In a futuristic computer control room housed within a Victorian-era house, an emergency is in progress. Computer technicians scurry around from instrument panel to instrument panel trying to make sense of the readings. A tannoy announces a phase one base evacuation. The technicians are becoming more and more desperate.

Into this chaos comes the leader of the team, a middle-aged man called Clent. He is gruff and short-tempered but always on the side of order. He clearly commands respect and obedience despite some physical problems. He walks with a cane on one side.

The technicians are controlling an apparatus called an ioniser whose power output has dropped to dangerously low levels for no discernable reason. Clent is furious and asks for an explanation. His number two, Miss Garrett, can offer no answer, nor can she find a way to fix it. This apparently is not the first time this has happened, but it could be the last. Clent orders all circuits to be switched over to computer control, refusing to evacuate until they've done all they can.

Miss Garrett wants to try and hold it manually and reports some success, backed up by an inhuman computer voice which announces momentary stabilisation. This at least will give them some breathing room. Miss Garrett tells Clent they need Scientist Penley to fix the ioniser but Clent bristles at this suggestion. Penley is no longer a member of this team and she is to make the machine work. She accepts this with an acid reply.

In the lull, Clent sits down at the computer terminal and gets a report from all ioniser bases all over the world. The ionisers are being used in concert with the world computer to contain a massive glacial advance from the poles on every continent in the world. All bases are steady or showing some improvement, except here at Brittanicus Base in Europe. Glacial advance is imminent.

Clent knows what this means. All the bases must work together on the agreed world programme. If not, the glaciers will advance and sweep over the civilised world, wiping out 5000 years of history in a mountain of ice. In two hours the Brittanicus ioniser will be useless unless they can do something to fix it.

Phase two evacuation is announced and Miss Garrett becomes more concerned. She reminds Clent that Penley was the ioniser expert but Clent clearly found him a bit crazy. Penley is clearly no longer on the base and Clent is still angry at this. He asks for Arden, another member of the team, only to be told that he is still at the glacier face on a project. Clent activates the communications equipment to try and contact him and apprise him of the dire situation.

Arden, along with assistants Walters and Davis, are drilling in the ice face in the midst of blowing snow and biting cold. They are embedding seismic instruments in the ice but Walters draws their attention to something he's found inside the ice face. At first they think the dark shape frozen inside could be an animal - which has happened before - but a closer look reveals some sort of helmet. This is some sort of man. All three scientists are astonished and intrigued.

Arden chooses to ignore the incoming communication from Leader Clent and to concentrate on whatever is in the ice. He is still unaware of the danger at the base. Clent is furious when Arden does not respond, certain he is being willful and derelict in his duties. But he has more important problems when Miss Garrett reports that ioniser power is slipping dangerously once again.

Outside in a snowswept wilderness, the TARDIS materialises. It is lying on its back on a hill and slides dangerously once it is solid. There are cries from inside as it does so. Once it comes to rest, the doors open outward and upward and the Doctor's head pops out, followed by Victoria and Jamie. The Doctor calls it a blind landing but does not apologise. Jamie is quite concerned as he thinks they've landed further up the mountain in Tibet where they've just come from.

The Doctor climbs over the edge of the TARDIS and drops down onto the ground. He helps Victoria down whilst Jamie climbs out on his own. Jamie's knee nearly crushes the Doctor's hand as he does so. Once they are all out, they are drawn to a curved white wall in front of them. Victoria thinks it a wall of ice, but the Doctor realises it's plastic. A plastic dome covering something very large underneath.

As they watch, a door opens in the dome and two men come out. They are wearing furs and are both dirty and disheveled. An alarm can be heard from inside. The smaller of the two men does not care about any of this, but the taller man does wonder what can be wrong. They hurry off, both carrying several boxes of supplies clearly looted from inside the dome.

The Doctor and his friends stay out of sight until they are gone and then approach the door. The Doctor finds an opening mechanism and they enter. The dome appears to be covering a large Victorian house. The door closes behind them.

In the midst of the chaos in the control room, Arden contacts the base. Clent breaks off to speak to him, ordering him to return to the base. Arden does not seem to share Clent's panic, even when he learns that the ioniser is on the verge of disintegration. He wonders if Penley's ears are burning and Clent is angered at this. He promises a full inquiry into Arden's delay.

Arden announces his discovery - a man frozen in the ice. He is determined to bring the body back with him, despite Clent's orders to the contrary. Arden fakes some interference in the signal and breaks off, leaving Clent furious.

The Doctor, Jamie, and Victoria have found beneath the dome a fantastic sight - a Victorian era home perfectly preserved. It is Brittanicus base, housed in an antique and covered with a state-of-the-art plastic dome against the cold and snow. Victoria is well pleased at this familiarity.

Phase three evacuation is announced and the Doctor becomes worried. A worker passes by and pins badges on the three of them, identifying them as scavengers to be evacuated. Jamie is incensed at the label but the Doctor has moved beyond that. He hears the sounds of electronic machinery in an adjoining room working overtime. The pitch is all wrong, indicating severe trouble, and he decides to go in, taking an impish joy in being able to help out.

Clent hobbles round the room, getting readings from all of the machinery, not noticing the strange little man shadowing him and taking in the readings as well. All of the readings indicate severe trouble and the Doctor is becoming very worried indeed. Finally he taps on Clent's shoulders to offer help but all Clent sees are scavengers in his control room.

Chaos breaks out as Clent calls for security, but the Doctor assures him that he's no scavenger. He announces that in 2 minutes, 38 seconds, the ioniser will explode! Clent cannot believe he can know this from the readings alone but Miss Garrett knows they cannot afford to ignore him. They must act.

The Doctor goes into action, running around the room from one instrument station to the next ordering very irregular changes and corrections to the machinery. Miss Garrett and the other technicians obey out of sheer terror and before long the Doctor has them cut in the computer stabilisers and the out-of-control machine hum fades away. The Doctor mops his head with a handkerchief, satisfied that he has solved the problem for the moment. He innocently suggests that an expert, however, will be needed to put it all to rights.

Clent is stunned at the Doctor's feat and refuses to believe it. He doubts the Doctor's prediction of an imminent explosion but when he runs the earlier readings through the computer, it confirms the Doctor's diagnosis almost down to the second. Clent has a new respect for this strange man.

Miss Garrett reports half power on the ioniser which is good enough for now. Clent is pleased and starts to ask the Doctor where he's come from but suddenly he staggers, worn out from a 10-hour duty shift and constant emergencies. His assistants help him out of the control room and toward the medi-control centre for some relief. The Doctor, Jamie, and Victoria go along on his orders.

Arden and his men survey their find, now carved out of the ice face and housed in a large block of ice. The creature is bipedal and very tall, covered in green scaly armour and a helmet. Arden thinks it looks pre-Viking but it must hail from before the first ice age and there was no such civilisation. Walters dubs it an "ice warrior" and suggests hurrying it back to base before the weather turns. They call out to Davis to bring the air sled closer.

Nearby, the scientists are observed by the two scavengers from the base. The smaller one with the Scots burr - known as Storr - is virulently anti-science and derides whatever it is Arden's doing. Archaeology is only poking around with dead things to him. The other man, though, shows interest despite himself. Storr is suspicious but his partner says it's only in his nature to ask questions. He knows Arden, and Clent, and has had enough of them. This is the AWOL scientist, Penley. He has gone scavenger in protest to Clent's leadership style.

They've seen enough and decide to leave. However, they must stay put for a moment as Davis passes nearby. Suddenly there is a rumble and a crack. An avalanche!

Davis is swept away by the snow, dead far below. Storr catches a glancing blow and is knocked down hard. Penley races to him, finding Storr alive but with a broken arm. Feisty as ever, Storr wishes to leave the area before they're caught, despite his injury. Penley helps him up and they hurry off.

Arden and Walters venture out from the crevice in which they've wedged themselves and see that the avalanche is over. However, there is no sign of Davis. They do not as yet know the worst and Arden jokes that Clent will be very angry if another man is lost. Walters suggest that Arden "set his warrior" on Clent if there's any trouble!

In the base's medi-control centre, Clent rejuvenates himself in the vibro-chair. The Doctor has offered to help Clent with the problem but the leader seems concerned over the Doctor's lack of credentials and his lack of knowledge of the current world situation. Thinking quickly, the Doctor explains that he and his friends have been on retreat in Tibet until recently and are unaware of what's been happening. Clent seems to buy this but puts the Doctor to a test. If he passes, he can stay and help. If not, he will be evacuated to Africa.

The test is high-pressure. Clent explains that the world's major continents are threatened with destruction due to the glaciers of the second ice age. The Doctor must come up with the solution in 45 seconds! The Doctor rattles through possible causes until he finally finds the answer: a severe drop in the carbon dioxide level of the atmosphere. With that comes the solution at the last second: ionisation. Clent is duly impressed.

For his confused young companions, the Doctor explains the need for carbon dioxide to maintain global heat. Clent adds that the growing population of the Earth took over most of the farmland, which was not needed due to artificial food sources. The amount of green plants was reduced to an absolute minimum, but it has turned out to be too low. Suddenly one year there was no spring and the polar ice caps began to advance. There is a bit of sadness in his voice.

Miss Garrett explains that the ionisation process is a way of reflecting and intensifying the sun's rays onto the ice in order to melt it. However, precise control is very difficult at ultra-high temperatures and they must be very careful to avoid global flooding. The world's computers are working on the problem of control but lack certain data. The solution, she is certain, will be forthcoming. That confidence is not readily apparent in the attitude of the base personnel. Right now, the glaciers are barely being held back and this base is in danger of letting the ice come through. Europe will be swallowed and the entire world programme will be endangered.

Clent says that his senior scientist, Penley, is missing and that he would like the Doctor to join the team to help. The Doctor agrees to try. Clent is appalled to learn that the Doctor doesn't really use computers, which he believes are the only true solution to the world's problems, but the Doctor says he will think about it. Miss Garrett tells him proudly that the base is completely computerised to eliminate risk of failure. All decisions and actions must conform to the common good and this is the only way. The Doctor is sceptical to say the least.

Arden arrives at the base with his warrior on a trolley. It is attracting a good deal of attention as he pushes it through the corridors and into the medi-control centre. Clent bursts out with invective aimed at Arden's deceitful tactics for staying out at the glacier. However, he falls silent when he sees the block of ice and the creature inside.

The Doctor is taken with it as well, but he is also puzzled. Clearly the creature has been frozen in the ice for centuries - since the first ice age - but the helmet seems very wrong for the time period. He knows, even more than Arden, that such a civilisation did not exist in those times. Arden believes he's discovered a previously unknown civilization but Clent refuses to let the discovery distract them from the task at hand. The daily ioniser team planning conference is to take place in 3 minutes and 15 seconds. Everyone must be there, despite the disruption. The group clears out of the centre, leaving only the Doctor, Jamie, and Victoria to look at the frozen warrior.

Arden has set up an electrical device designed to melt the ice around the warrior. It is working quite quickly even when it's at low power. The Doctor is further puzzled when he gets a good look at the creature's helmet, for on it is an electronic connection. He becomes very worried at this and races off to find Clent, leaving Jamie and Victoria here and warning them not to touch anything.

Jamie and Victoria banter idly about the length of the female scientists' hemlines while the ice warrior begins to thaw behind a screen. Jamie approves of the short skirts but the proper Victoria is scandalised. Behind them, unseen, the ice continues to melt away from the creature. It is happening very quickly now.

Unseen by Jamie and Victoria, the creature is now completely free of the ice. And it is alive...!

Victoria is stunned as the warrior stands before her, very much alive. It is very tall and imposing and breathes with a hiss. She calls out to Jamie but she is far too late. The warrior clubs Jamie down and drags Victoria from the room.

The ioniser planning meeting is dominated by talk about the Doctor. Clent is keen to have him join the team, but the others are less impressed with his strange manner and odd behaviour. Clent believes he is as smart as the computer but wishes to submit the question to the computer before deciding. The computer's assessment is thus: "High IQ but undisciplined for our needs. Present evaluation - to be used on research projects but could be highly obstructive in certain situations."

Suddenly, the undisciplined Doctor obstructs the meeting by bursting in without knocking. Clent tries to keep him out of the meeting but the Doctor will not be dissuaded. What he has to say is of the utmost importance. Arden guesses immediately that it has to do with the warrior.

The Doctor finally reports the electronic connection he saw in the warrior's helmet and the conclusion that the creature must be extraterrestrial. He is certain it is not an unknown civilisation from the early ice age but an alien astronaut wearing a sophisticated space helmet. Clent does not believe this, thinking that the Doctor is jumping to a pretty fantastic conclusion, but the Doctor insists. If this creature is an alien, it must have come to Earth by spaceship, a spaceship that is most likely still frozen in the glacier somewhere. The creature is intact and shows no effects from a crash so it must have landed. Arden is very excited by this, knowing that it was his find, but Clent and Miss Garrett grasp the larger implication and the danger that the Doctor immediately saw.

If that spaceship is atomic powered, its propulsion unit could be detonated by the intense heat of the ioniser. The plan for global containment of the glaciers could result in a nuclear wasteland being created right in the heart of Europe! However, not using the ioniser is not an option either. Even at half power, the glacier would soon advance and the world programme disrupted.

Tempers flare and Clent demands more information before any action can be taken. The chaos rises when Jamie bursts in, looking much worse for wear. He is ranting about the warrior coming to life and taking Victoria, stirring everyone into a frenzy. The room empties as they all head for the medi-control centre. Arden is wide-eyed with disbelief. His discovery.

The Doctor investigates the table and the power unit used to melt the ice around the warrior. He determines that that is what brought the creature to life. Arden used a low voltage in order to melt the ice slowly, but the low resistance due to the warrior's armour created very high temperatures which must have shocked it back to life. All the scientists are astounded. Jamie, however, doesn't care a bit about how it happened. What's most important to him is that Victoria's in danger. The Doctor agrees and they prepare to search.

Clent issues an alert throughout the base, assuming that they are still in the base. He refuses to consider a search outside the base as he cannot spare the manpower. The Doctor is angered by this, fearing that this short-sightedness may threaten Victoria's life. Clent responds to the Doctor's show of emotion by consulting the computer for an impartial opinion, his standard response. The Doctor and Jamie know they are only wasting time.

The giant reptilian warrior has taken Victoria to a storage room not far from the medi-control centre. He is a bit disoriented after his long sleep and wishes to get some answers. However, the nervous Victoria pelts him with questions first. The warrior's name is Varga and he comes from the red planet, Mars. But that is all he will answer. He brandishes a small tube mounted on one of his pincer-like hands and demands some answers.

Varga learns from Victoria that he has been frozen in the ice for thousands of years but has a hard time believing it. He explains that he and his men were in a spaceship that crashed at the foot of the ice mountain. When they came out, they were buried in an avalanche. That is all he remembers. He vows to find his men and thaw them out. They will all return to the red planet. But he knows he cannot free them by himself. Victoria will help him.

Victoria, scared by this pronouncement, suggests Varga get help from the scientists. He rejects this, certain he would only be kept as a scientific curiosity. He vows to revive his men with Victoria's help. This will give him strength in numbers and they can then decide whether to return to Mars or to conquer Earth!

The computer renders its verdict on Clent's inquiry: ionisation should take priority over the search for Victoria as only a limited number of lives are "expected to be lost". However, the possible presence of an alien spacecraft is of concern due to its potentially atomic power source and the computer determines this is a priority. A search must be conducted to confirm the presence of atomic material. The duty schedules are automatically rearranged to free Arden to search for the warrior and his ship.

The Doctor is pleased at this response, but suggests that Jamie be allowed to help Arden. He is not a member of the base's staff and is additional personnel. Clent agrees, as long as the Doctor stays to help with the ioniser. The Doctor agrees. Clent orders Arden to head out immediately, making sure he knows that finding the spaceship is the top priority. To him, Victoria is expendable because the computer said so.

Penley has taken the injured Storr back to their hideout, a plant museum not far from the base. It is very near the glacier and has long been abandoned. All of the plants are still there though, part of the few that are left in the world. The plants provides them with warmth and food. Storr's wounded arm has become infected due to his stubborn refusal of medication and Penley is beginning to worry. Storr is still stubborn, even as he weakens.

Penley picks a tomato and gives it to Storr. They reminisce about the way the world was before the plants were taken away. It is that very situation which has given rise to Storr's strong stance against science. By killing the plants, scientists brought the new ice age upon them. As if to reinforce this idea, there is a rumble from the mountain. The glacier is close and ready to move at any time, crushing the museum. Penley begins to gather his things to go out. He is going to the base for some drugs to help Storr's infection. If not, he's as good as dead.

Victoria tries to explain the device which brought Varga back to life. Not being very scientific - and hailing from the 1860's - she has a very difficult time. However, Varga seems to understand the resistance and the heat. He wishes to return to the medi-control centre to fetch more of the power units and demands that Victoria take him there. He assures her he can take care of anyone who may try to stop them. He brandishes the gun attached to his arm, a sonic weapon that can burst the human brain with noise. The frightened girl has no choice but to help him.

Jamie and Arden have returned to the site where Arden excavated the "ice warrior". Believing the spaceship to be buried in the glacier, they have surmised the warrior might try to return there. There is no sign of him, nor of the spaceship, but there is definitely something there causing havoc with Arden's radiation sensor. Not a good sign.

Arden reports to Clent on his wrist communicator. The leader has no interest in whether they've found the warrior or Victoria, only about the presence of the ship. Clent wants him to dig into the ice to find the ship but Arden does not have the proper tools. Clent testily orders Arden and Jamie back to base to get the proper equipment. He will brook no argument.

Jamie is not amused at Clent's hard-heartedness. Arden speculates that Clent should have been born a robot for all the good his feelings do him!

Victoria leads Varga toward the medi-control centre, glancing furtively down the corridor. She is quite disappointed that there are no base personnel to be seen. Where can they all be? They move toward the proper door, unaware that they are being observed...by Penley. He has snuck into the base and is astonished by what he sees. He follows at a distance.

In the control room, the ioniser is back at half power and holding steady. Miss Garrett is pleased, but the Doctor knows that this will not last. He asks again why there is no expert here to fix the machinery and is stunned when Clent says that he chooses not to get another expert. He admits that he is afraid of admitting failure with the world - and his own sterling reputation - on the line. He hand-picked the team but says that choosing Penley was a mistake. He is brilliant in regards to the ioniser but is "hopelessly temperamental". The Doctor wonders if his temperament is not just a sign of individuality but Clent rejects this. When Penley walked out, he made himself criminally responsible for whatever happened to the ioniser. His petty "protest" against Clent may endanger the world.

The Doctor infers that Penley is still needed here but Clent rejects this as well. All he needs is an equivalent brain, but there may not be sufficient time for anyone else - however brilliant - to acquire the ioniser knowledge that Penley already has. The Doctor agrees to help as best he can, even though he suggests that Clent might try trusting people instead of the computer. Clent huffs off to the medi-control centre to see if the Doctor's working space has been prepared.

In the medi-control centre, Victoria has finally found the power pack used to bring Varga to life. It is a large black box with wires. Satisfied, Varga takes the unit and grabs Victoria to leave. He is ready to go to the glacier and free his men. As he turns, he sees Clent barring the door. The leader is stunned at the sight of the living warrior and reacts much too slowly. Varga knocks Clent unconscious with the heavy power unit and drags Victoria away. She screams that Clent is dead.

As soon as they are gone, Penley enters and examines Clent. He makes no move to go after Victoria or the warrior. Unseen, the Doctor approaches, watching Penley with interest. Penley notices him as he prepares to administer smelling salts to the unconscious leader. The Doctor addresses him pleasantly, saying that the salts are just the thing to bring him round. The Doctor has deduced who the man is.

The Doctor wonders if Penley was the one to knock Clent out. Penley laughs, saying he's always wanted to, but tells the Doctor about the warrior. The Doctor is shocked and asks about Victoria; he cannot understand why Penley didn't do something to help the frightened girl. Penley responds, saying he couldn't afford to get caught. His friend is dying and needs the drugs he's stolen - he faces certain death without them - and Victoria looked like she was fine if very scared. He couldn't risk his freedom for her. The Doctor can see the logic in this but wants to get after the creature immediately. Penley offers to help but the Doctor knows he will be caught if he does. He decides to get out quick.

Before Penley goes, the Doctor tells him that he is needed here, desperately, and that the problems of this base are the problems of the world. He is trying to instill a sense of obligation in the man but Penley doesn't buy it. All that is sacred to him are his own thoughts and individuality. He will not sacrifice them for Clent or anyone. The Doctor is dubious, to say the least.

Penley just makes it away before Miss Garrett, Jamie, and Arden arrive in the centre. The Doctor wakes Clent up with the smelling salts. Miss Garrett is already alarmed because of a report that the warrior has broken out of the base, taking the girl with him. The Doctor learns that the warrior had a power pack and he also becomes worried. He realises that the warrior is planning to find and revive more creatures like himself. This is something that no one else had thought of.

Jamie is ready to get after them now, hoping to rescue Victoria before they reach the glacier, but the Doctor holds him back. He thinks that the warrior wants them to go after Victoria. She is a hostage to bring others to him. The Doctor decides to wait until the morning to search, much to Jamie's consternation. Arden backs him up, refusing to even consider going out on the glacier as night falls.

Using a homing device, Varga easily finds the location of his men in the ice face. He lets loose of Victoria, warning her not to run, then he turns to ice. He activates his sonic gun and begins excavating.

Penley has returned to the plant museum and administered the drugs to Storr. The small man sleeps now, not knowing what Penley has gone through to save him. Penley decides to go back out on the glacier to do some "hunting". There is something very strange going on out there and he wants to know what it is. The Doctor's words have also gotten to him as he's thought about them. He now seems to believe that he might really be needed at the base. If so, he might actually go back there.

Varga is pleased as the ice begins to melt around the first of his warriors, Zondal. After many centuries in the ice, he will soon live again. Victoria is horrified, watching wide-eyed and silent.

Not far away, Penley watches from hiding...

The next morning, Jamie and Arden suit up to go back out to the glacier to try and find Victoria. The Doctor warns Jamie to keep an eye on Arden, a single-minded scientist who doesn't seem to realise just how dangerous the warrior is.

Clent tries to make certain that Arden knows his mission - to find and report on any alien spacecraft and its propulsion unit only - not finding the missing girl despite Jamie's objections. However, Arden is having trouble focusing as he believes that all of this is his fault. His curiosity got Davis killed and brought this creature to life. He is being very hard on himself for the current situation. Clent, awkwardly, tries to reassure him. Emotions are clearly not his strong suit. Jamie assures them all that he will make Victoria's safety a priority, despite Clent's insistence on focusing on the spacecraft only. With that, Arden and Jamie are off.

Clent then turns to the Doctor, brisk and businesslike, ready to put him to work. However, the Doctor bristles under his authoritarianism. He reminds Clent that he just a guest here, helping out of his own choice, and that the base's regulations don't apply to him. Clent, disparaging, likens the Doctor to the individualistic Penley. He hopes that the Doctor doesn't suffer the same "breakdown". This is clearly a mischaracterisation of what happened and they both know it. The Doctor pays it little heed although Clent tries to justify the use of the word.

As the Doctor starts to work, Miss Garrett is nowhere to be found...

Penley has returned to the plant museum, badly shaken by what he has seen. He tells the recovering Storr, who is sceptical. Storr is also dismissive of the drugs which saved his life, despite the evidence of his own eyes. Storr is a Luddite tried and true. Suddenly there is a sound outside. Someone is here. Penley hides Storr and then goes to the door. He is surprised indeed to find Miss Garrett.

Apparently he was not unobserved when he left the base last night. Miss Garrett followed him and has come here to beg him to return to the base. She says he is the only one who can help them to make the ioniser work again. But Penley knows the real reason she's come: Clent is faced with reporting his failure to World Control and cannot do it. He has not sent her, but Miss Garrett hopes to help Clent by getting Penley to return. He's not interested in helping.

Miss Garrett presses him further, perhaps banking on a personal relationship they once had and telling him that civilisation needs him. He can't just "drop out" and become a scavenger with the knowledge he possesses. Penley rejects this, saying that the "civilisation" Earth now supports is not worth saving. The "computerised ant-heap" that rejects individuals like him will pass with this crisis but people like him will still survive. Clent, whom he calls a "robotised human", will not survive a calamity like this for the very reason that he cannot adapt and think creatively. If Penley was to "save the world", as Miss Garrett grandly puts it, it would only be a world he does not accept or appreciate.

Faced with this stern denial, Miss Garrett abandons reason and produces a tranquilliser gun. She will force him back to the base. However, Storr has been watching all of this and leaps from hiding at the sight of the gun, knocking it from her hand. Penley retrieves the gun.

Miss Garrett tells him acidly that she will return to the base and await "the inevitable disaster" and Penley is content to let her. However, Storr is certain she'll report them and he'll be caught and sent away to Africa. He tries to take the gun from Penley but is too weak. Penley trusts that she will not give them away. She says she will not.

Miss Garrett tries one last tactic, trying to get Penley to at least think of his safety. There's the glacier bearing down on them and then there are the warriors on the loose, but Penley is certain he can look after himself. He sets her loose, telling her he'll not come back to the base, but he does give her a clue. He tells her to look up his notes on something called the "omega factor". With a depth of sadness on many levels, Miss Garrett leaves.

Storr is surprised to have heard Miss Garrett confirm the existence of the "ice warriors". Penley decides he must find out more about them and suits up to leave again.

Victoria is now surrounded by the warriors, all alive and all very menacing. She plucks up the courage to ask Varga to release her. Of course she is refused. Varga charges his second-in-command, Zondal, with the task of finding their buried spaceship. He is to excavate a cave around the entrance of it to act as a trap for any humans who may be foolish enough to attack.

Victoria protests that her friends would only want to help the warriors, but Varga rejects this. She is a mere frightened child and will say anything to save herself. Besides, she is useful to them as bait for their trap.

The warriors find the ship in mere seconds using their instruments, and activate their sonic guns to begin excavating into the glacier. Penley watches again from a distance.

The Doctor, meanwhile, has made himself quite at home at the ioniser base. He has printouts and notes scattered all around and he scurries between the piles trying to make sense of all the data. Clent is chagrinned at the chaos. The Doctor rejects his offer of a computer but does gratefully accept a pencil.

He has gathered much information but there still seems to be something missing. Clent provides him with readings from the other ioniser bases and Miss Garrett, quite tardy, provides him with Penley's notes.

Looking over the notes, the Doctor is immediately drawn to the "omega factor". Excitedly, he puts the information together with what he already has and believes to his joy that he has come up with the solution. Clent is surprised but relieved, and he's still sceptical of the Doctor's working methods. The Doctor assures him the solution he's come up with will work and explodes with anger when Clent insists on checking it on the computer. He leaves to go and set up a simulator run, leaving the Doctor quite angry behind him.

The Doctor praises Miss Garrett's "inspiration" in looking up Penley's notes and ascertains that she regards Penley very highly indeed. He says, dryly, that he's glad to see Penley still has some friends in the base.

Arden reports in via wrist communicator and tells Clent of the new cave excavated in the glacier. He also reports a metal door at the back of the cave. He is sure it is part of the warriors' spacecraft. Clent agrees and urges him to complete his readings on the ship's propulsion unit and return to the base quickly.

Unbeknownst to them, the warriors have overheard the entire conversation.

Arden prepares to take the readings, awed by this up close contact with an alien life form, when a group of warriors appear from hiding and open fire. Arden takes the brunt of the blast, but Jamie is hit too. Both of them fall to the ground. Victoria bursts out and sees the bodies and is horrified. She accuses them of killing her friends. Varga tells her it was "necessary" and has the sobbing girl bundled back inside.

Varga is pleased at this first test of their might. If any more of the humans come, and he believes there will be more, they will be dealt with in the same way. He is glad that he kept Victoria as a hostage. She is useful to them alive a little longer yet.

Varga charges Zondal with yet another task, this one most important. He must make the propulsion unit of the ship functional again, and he must do it quickly. They both return to the ship.

Hiding in the cave, Penley has watched all of this. Once he is alone, he goes to examine the bodies. Arden is dead but to his surprise the boy is still alive. He must have been shielded from the blast by Arden's body. He is certain he can help and sets about taking Jamie away from the cave.

Clent tells the Doctor that the computer simulation is about to begin. They should know soon whether the Doctor's solution will fix the ioniser or not. The Doctor has already moved on and is worrying about Jamie and Arden. They have not reported in with the results of their readings and it has been too long.

Clent calls Arden on the videolink but receives no reply. The Doctor is certain this means trouble. Something has happened to them. He is becoming more and more agitated, worried about his companion, and he has little time for Clent's machine-like concern for the ioniser program should Arden be dead. The Doctor is about to chastise him for his inhumanity when Miss Garrett bursts in. She reports that the simulation was a complete success. Clent suddenly becomes a new person - arrogant and self-assured - speaking grandly of the victory of science over nature. Europe is saved thanks to him. What a triumph!

The Doctor is appalled, explaining that two of their friends could be dead on that mountain this very minute, but Clent has no time for "sentiment". He laments the loss of a valued colleague but that is all the emotion he can muster. He sounds like an automaton. In the success of the moment, however, Clent has lost sight of one very important fact. The ioniser may be back to full power, but the Doctor reminds him that they dare not use it until they find out about the propulsion unit of the alien spacecraft. Clent's façade crumbles and he returns to the videolink, trying to get Arden to respond.

Storr resents having to look after the injured Jamie, certain that no good can come of having base personnel traipsing through their hide-out. Jamie regains consciousness with a splitting headache. He is saddened to learn that Arden was killed by the warriors and is acutely aware that they've failed in their mission.

Penley reports that Victoria is still alive and Jamie rallies. He is ready to go back after her right now, begging Penley and Storr for help, but he is still far too weak to go. Jamie protests and tries to rise but his aching head gets the better of him and he drifts back into unconsciousness, mumbling about time running out for Victoria.

In the excavated cave, Victoria sneaks out of the spaceship. Somehow she has escaped her captors. She finds Arden's wrist communicator and looks it over. She knows what it's for but has no idea how to make it work. She begins stabbing at the buttons.

Zondal sees this on his scanner and becomes concerned, but Varga puts him at ease. He has allowed her to escape.

Victoria manages to activate the videolink and the Doctor sees her image on the screen. She is glad to have reached him but she is still very frightened. She tells him about the warriors shooting Arden and Jamie but she cannot tell their fate. Arden is still here in the cave and he is clearly dead but for some reason Jamie's body is now gone. She is becoming hysterical as she jumbles up her facts.

Clent bursts into the conversation and badgers her for details. He wants to know about the spaceship. Victoria can barely comprehend what she is being asked. All she knows is that these warriors are ruthless alien killers and they'll stop at nothing. She is afraid for her life and begs for the Doctor to help her.

Varga sees what Zondal is doing and deactivates the gun before he can fire it. The girl is not to be harmed. Zondal fears that she will give away too much to the other humans but Varga is using her to bring the others here. He has need of information.

Victoria continues to speak with the Doctor on the videolink, telling him that the warriors are from Mars, but they are interrupted by Clent. He badgers her to tell him what type of engines the ship has but his bluster just makes the girl more nervous. The Doctor calms her and asks if she can at least describe the engines. She says she can but suddenly the link is swamped in interference as the glacier rumbles all around. Her face comes back into view and she starts again to answer the question.

Varga thinks she has given enough information and orders his men to fetch her. Zondal again wishes her destroyed but Varga needs some answers. He wants to know why the humans are so interested in their ship's engines.

Victoria tells the Doctor that someone is coming and that she'll have to move. She says she can see the base from the opening of the cave and that she'll try to reach it. The Doctor wishes her luck, staying positive for her sake, and then cuts the link.

Victoria hides herself in a fissure in the ice cave, hoping that the approaching warrior will not see her. There is another rumble and part of the ceiling caves in. Victoria is all right, but the warrior is still approaching, guided by a tracking device.

The Doctor, very worried about his companions, uses an advanced chemical dispenser to dial up some old fashioned H2O. He feels better once he's had some and turns his attention to the situation at hand. Jamie is missing and Victoria is a fugitive from the warriors and they are still not certain what kind of propulsion unit the alien spacecraft has. As he talks, he dials up another formula on the chemical dispenser. He comes up with a phial of ammonium sulphite which he says will help them with their problem.

He tells Clent and Miss Garrett that he plans to go to the alien spacecraft himself and to find out what type of engines it has. The ammonium sulphite is a weapon he plans to use against the aliens if needed. They are from Mars and the chemical will not be very pleasant for creatures used to a nitrogen atmosphere. It will make a handy toxic gas should the need arise.

Clent is not keen on allowing the Doctor to go, having already lost Arden, but the Doctor insists he is better prepared than Arden. He knows what he is facing and won't fall into the same trap. Clent orders him to stay put, but the Doctor refuses to be commanded. He is not needed here now that the ioniser is fixed, but the information on the alien propulsion system is of the utmost importance. He is the best person for the job and he assures Clent that he and his staff can handle any further problems with the ioniser. Reluctantly, Clent agrees.

The Doctor takes only the ammonium sulphite and a wrist communicator and nothing else. Clent insists he needs a weapon but the Doctor disagrees. He's not going there to fight a duel! He says he's going there to talk to them and he believes they will not kill him out of hand. If they try, he can use the gas. Miss Garrett believes he is going to willingly become their prisoner. To her astonishment, he says that is exactly what he intends to do!

Victoria is spotted by the warrior and she runs. However, she finds herself in a partially-finished part of the cave which is blocked off by a window-like coating of ice. She bashes her way through the ice and thinks she is free. However, she has dropped her communicator along the way. The warrior bursts through the wall and marches straight past the girl, who has hidden again in a crevice. Once he is past, she reaches for the communicator, but the warrior has heard her and turns. He grabs her just as another rumble shakes the mountain and the roof caves in on top of them.

Penley hovers over Jamie looking worried. The lad's fever is gone and it looks like he'll pull through. He is concerned over the nature of the weapons the creatures used on him. Storr, characteristically, thinks they are just more evil science.

Jamie awakes, disoriented and raving, and Penley and Storr must calm him down. The pain in his head subdues him and Penley tries to apply a tranquillising pad to help ease the discomfort. The suspicious lad resists, endearing himself to Storr. Penley tells Jamie again that he saw Victoria alive and that she is being held prisoner in the alien spacecraft. He tells Jamie that the warriors are not from this time or from this planet. He finds it all difficult to understand.

Jamie is ready to get up and go and find Victoria. Penley tries to stop him but he finds an ally in Storr. Storr thinks Jamie is an ally in the anti-science camp. However, when Jamie tries to stand he cannot feel his legs. Penley is certain it was the weapon used by the creatures that did this. It must have affected his central nervous system. Jamie is stunned to realise he may be permanently paralysed.

In the warriors' spaceship, Varga is concerned that Turoc has not returned with Victoria. Varga still needs to use Victoria to draw a scientist from the base. The ship's engines have proved to be functional but they lack sufficient nuclear fuel to take off. The centuries have taken their toll on the ship. Without that fuel, they are helpless.

But there is another worry in the warriors' minds. Varga has realised the purpose of the base - to melt the ice. If they succeed there may be floods which will swamp the ship. It may be necessary to attack the base with their weapons in order to stop the ice being melted before the ship is ready to leave.

Outside in the cave, Victoria has survived the roof collapse, but Turoc has not been so lucky. He took the full brunt of the cave-in and has been crushed under the ice. However, his pincer-like hand still grips her wrist tightly and she cannot get free. Another rumble echoes all around and more of the roof falls in around her. She is all right, but she knows she cannot stay here for long. She calls out at the top of her voice, not caring who or what comes to her aid. She must get out of here.

Penley and Storr have used all their limited resources to try and help Jamie. There appears to be no cause for his paralysis, let alone a cure. As Jamie sleeps, Storr decides that there is only one way to get an answer - befriend the aliens. He puts on his coat and boots.

A stunned Penley tries to talk him out of this foolish act. The aliens are ruthless killers who place no value on human life. Storr thinks they killed only out of self-defense and he is certain he can talk to them. He refuses to consider taking Jamie to the base. He will no longer consider being any part of that computerised world, even to help Jamie. He would rather take his chances with the aliens. Nothing Penley can say will stop him and Storr leaves the museum.

Storr makes his way to the cave, hearing Penley calling out to stop him. Penley hides, pressing himself against the ice and clutching an icy rock with which to attack Penley if necessary, but when he hears Victoria's plaintive cries, he drops his weapon and goes toward the girl.

Penley follows the same path, determined to stop Storr. A roof collapse diverts him along a different path and he encounters the Doctor near the door of the warriors' spaceship. The two regard each other with interest. Both men seem to understand one another due to their similarities.

The Doctor says he has not seen Storr and Penley despairs that he is too late. The Doctor offers to help but Penley wants to be sure the strange little man can be trusted. He asks why the Doctor didn't give him away at the base. The Doctor says he had no reason to. He doesn't believe Clent's stories about Penley and prefers to make up his own mind about people. Penley accepts this. He assumes that Jamie is a friend of the Doctor's and offers to take him back to the plant museum. The Doctor accepts, grateful for the help.

Storr reaches Victoria and releases the alien's grip on her wrist. She is grateful for the help, but a little dubious at the sight of the scruffy little man. She tells him that she was trying to escape from the ship when she was caught. She says the warriors are evil and she thinks they see the ioniser as a weapon and may want to destroy the base.

Storr hears what she says and takes it completely differently. He sees that the aliens are against the scientists, just like he is. Their reasons are immaterial. He, too, believes the ioniser is a weapon of sorts, a weapon that will destroy civilisation. Victoria is too glad to be alive to argue, and she is even more excited when she learns that Jamie is still alive. Storr says he's going for help but doesn't say where. There is another rumble of ice and the two hurry away from the weakened cave roof.

Zondal works to adapt the ship's weapon for use against the base. Varga watches approvingly. He is still concerned about Turoc's absence.

Storr leads a shocked Victoria directly to the ship's doorway. He tries to explain that the creatures will listen to him because he is against the base, just like they are. Victoria knows this is not true and struggles to get away from him. Her protests draw Varga to them. The girl shrinks back when confronted by the alien giant.

Victoria tells Varga of Turoc's death and Varga grabs her. It is her fault that his warrior is dead. She is dragged inside, worried that she will now be killed.

Varga, joined by Zondal, turns to Storr. He announces that he is against the scientists at the base, denouncing their evil technology. He wishes to join with them in destroying them. The warriors see only a useless scavenger; he knows nothing of the technology that they need and has no value to them. Storr's tragic miscalculation is complete when the two warriors destroy him with their sonic guns.

They return to the ship to question Victoria.

The Doctor has examined Jamie and tells him happily that the shock to his brain administered by the sonic guns is only temporary. His legs will be back to normal in a matter of time. Jamie is relieved.

Penley pulls the Doctor aside, knowing that Jamie must receive more treatment before he can recover. The only place for that is the base. The Doctor says that he has something else he must do and that Penley must take him to the base. Penley balks, knowing he cannot face Clent alone. He has no emotions and will not accept his return. The Doctor however thinks that Clent's dedication to the ioniser mission will take precedence over everything else. He will accept his ioniser expert back, no matter the circumstances.

The Doctor tells Penley and Jamie that he is going to the warriors' ship to see if Victoria got away and to find out whether the engines are a danger to the ioniser programme. He knows it is dangerous but he hopes he can reason with the creatures. He knows there is no other way.

In the base, Clent can see evidence of the fifth ice surge today. The ioniser is not holding back the glacier at just half power and they daren't increase it for fear of a radioactive explosion from the alien ship. They are at an impasse and cannot act until the computer tells them what to do. It cannot do that until it has complete information. They must get that information from the Doctor.

Miss Garrett takes this all in somewhat calmly, figuring that Clent has some contingency plan if they do not get the information about the spaceship. However, it becomes clear that he does not have another plan, so dedicated is he to the computerised programme. He has pinned all his hopes on the Doctor. This does not comfort Miss Garrett.

The Doctor's face appears on the monitor of the communications unit. He is about to approach the spaceship. He leaves the communicator on so that Clent can hear what is being said.

Boldly, the Doctor knocks on the spaceship's door, asking to be admitted. The ship's weapon activates and the Doctor bravely ignores it. He decides to count to ten, not making clear what he'll do if they don't let him in.

Amazingly, the bluff works and the door opens. The Doctor goes in to find himself in a small antechamber. The outside door closes behind him.

In one wall, there is a video screen with Varga's face on it. The warrior demands to know who the Doctor is. Continuing with his brazen approach, he refuses to answer until he is addressed properly. He remains silent and defiant.

Varga then reveals to him that he is standing in the ship's airlock. Unless he agrees to answer all of Varga's questions within 10 seconds, he will reduce the pressure in the airlock to zero, causing the Doctor's body to explode.

The Doctor's façade crumbles and he becomes very worried indeed. Varga begins counting up the seconds and already the pressure is reducing.

The Doctor resists for as long as he can, but the reduced pressure is already affecting him badly. He knows he is fighting a losing battle and gives in. He agrees to answer Varga's questions and the air pressure is restored, although he says he doesn't like the warriors' version of hospitality! The Doctor says he is a scientist - of a sort - and he has come to talk to them. To help them. Varga is dubious, but he feels that the Doctor cannot afford to lie in his situation. He is allowed to enter.

The Doctor offers to help the warriors, but proposes some conditions. Varga scoffs at this until the glacier shifts around them and the ship rocks. The Doctor reminds Varga that it is the warriors who are prisoners. The ship is trapped in the ice and the Doctor says he can see that they are freed, despite their use of him and Victoria as hostages. Varga of course does not believe this; he still sees the ioniser as a weapon. The Doctor insists it is not and reveals that he knows how the ioniser works. This intrigues Varga.

Victoria is brought in and she runs to the Doctor. They are very glad to see each other safe. Victoria is also glad to learn that Jamie is alive. Varga cuts short the reunion and demands to know why the ship has not already been freed if it is such a simple task. The Doctor tells him there are "difficulties" but refuses to elaborate. He does not want to tip his hand regarding the ship's propulsion unit just yet. Varga is not pleased with his reticence and raises his weapon to force the truth from him.

Penley drags Jamie through the snow on a sledge toward the base. The lad is still unable to move his legs. Penley stops for a rest at the edge of the base perimeter. The sounds of wolves howling nearby disquiets them both. Penley thinks they're safe for the moment but the last stretch is open country. They'll be unprotected. Penley wishes Storr were here to help him or to at least watch their backs. Penley has only a tranquilliser gun to protect them. Not very adequate.

However, they decide to chance it and start to move from cover. To their horror, there is a bear very close by. There is no place for them to hide.

Varga menaces the Doctor, hoping to instill fear in him. But the Doctor is single-minded. The only thing he fears is the potential nuclear holocaust the spaceship's engines could cause and he reveals this to the warriors. Varga sees this is in his own favour. As long as the scientists believe that the ship could explode, they will not dare use the ioniser against them. But if that fear is eliminated, the ioniser could be used as the weapon he believes it is. The Doctor tries again to convince Varga that the ioniser is not a weapon, but the warriors' minds are closed. Even if it is not used directly against them, the ioniser will melt the ice and cause floods which will swamp the ship's engines and render them useless. It is a no-win situation for the Doctor and his friends.

Varga realises that the Doctor must have some sort of communication device and orders him to hand it over. In the base, Clent and Miss Garrett have overheard the entire exchange. The Doctor reluctantly hands over the wrist communicator which Varga takes in his pincer-like hands. Communications are lost.

However, the Doctor has managed to send one last piece of information to the base: at some point they must use the ioniser whether they hear from him again or not, regardless of the consequences. Clent and Miss Garrett are conflicted. If they don't use the ioniser at full power, the ice will advance and the world programme will be ruined. If they do, they risk a much worse global problem. And the computer cannot advise them without the information on the spaceship's reactor.

To add to their problems, the world computer announces that the Doctor's formula for ionisation has been adopted by all the bases in the world. A full-scale assault on the glaciers around the world is scheduled for six hours from now.

There is no way in the present situation that Brittanicus base will be ready to participate, dooming the effort. Anguished, Clent decides to try giving the computer the information they have and trying to get an answer. Miss Garrett protests that they do not have enough information but Clent insists. He is becoming desperate. The computer is his only hope and it seems a very long shot indeed but he dare not make the call himself.

Jamie and Penley watch the bear as it stands near them. It has caught their scent but has not moved as yet. They are becoming more and more worried. Jamie, tied into the sledge, cannot move. Penley will not leave him and despairs that his tranquilliser gun will make an impact on the giant creature... if indeed he can hit it.

Finally, it becomes clear they have no alternative. Penley fires at the bear, but misses. This angers the creature and it turns to attack them. There is nowhere to run. Penley screws up his courage and fires again as the bear lunges...

At the base, the computer renders its decision based on available information - set up for the Doctor's new ioniser equation and then wait for further information on the spaceship's propulsion unit. In effect, no decision is made at all. Miss Garrett had expected this and agrees with the decision. Clent, in a rare moment of doubting the computer, sees it all rather differently and he addresses the staff with his concerns.

The computer is so logical that it cannot take risks. It is also programmed to survive. At this juncture, any decision made by the computer could lead to its destruction - by the glaciers or by nuclear annihilation - and it cannot order its own destruction. By demanding a decision from it, they are asking it to commit suicide. Clent has realised that the machine is only playing for time.

Miss Garrett suggests evacuation of personnel but Clent says he could not face the negative world opinion if he were to run. For now he decides to follow the recommendation. They must prepare the ioniser and wait. Their fate is in the hands of the Doctor.

The staff do as they are ordered, taking some solace is the intense work of preparing the machine, but their potential fates weigh heavily on them all.

Penley has succeeded in tranquillising the bear, but not before being mauled pretty badly himself. His face is slashed and bleeding but he is able to get up. Jamie gives him encouragement, reminding him that the bear will be in an even worse mood when it wakes up! The two men hurry toward the base.

Varga has shown the Doctor the engine room of their spaceship, confident now that the Doctor cannot use the information. It possesses an ion reactor which the Doctor deems dangerous but not necessarily at great risk of explosion. He hopes now to be able to offer help to the warriors, using the ioniser to free them from the ice. However, Varga knows that this is not their primary problem. They still need fuel, and he plans to get it from the base. The Doctor at first refuses to tell Varga what kind of reactor the base possesses, but when Victoria's life is threatened, he tells him that what they need is there. Pleased, Varga quickly goes into action. He plans to use the ship's sonic cannon to attack the base and convince Clent that he must cooperate.

Zondal is placed on the sonic cannon whilst Varga, Isbur, and Rintan go to the cliff overlooking the base. Victoria cannot believe the creatures will kill the scientists who are only trying to help them. But Varga tells her ominously that his people "only fight to win".

While Clent and his staff implement the changes to the ioniser programme, Walters announces the arrival of Penley and Jamie. Clent is unwilling to be interrupted at first but when Penley says he has news of the Doctor, Clent has them brought to him.

Miss Garrett is pleased to learn that Penley has returned, seemingly of his own choice, but Clent is quite disturbed. Things seem to be going smoothly with the ioniser so his presence isn't a necessity anymore. If Penley tries to start trouble, Clent vows to deal with him. Penley is now an "outsider" and requires no special treatment.

Penley enters, supporting the paralysed Jamie. Everyone fusses around the lad at first, somehow finding it difficult to start the inevitable conversation. Clent seems to fear that Penley will become violent. Penley assures him that will not happen, willingly relinquishing his tranquilliser gun to Walters.

Clent and Miss Garrett seems upset that Jamie has been brought here. This base is not a first-aid post, regardless who the patient is. But news of the lad's paralysis does evoke concern. Jamie sidesteps all of this, trying to get someone to worry about the Doctor and Victoria, still trapped in the alien spaceship. Clent says there is nothing that can be done for them and this sets Penley off.

Penley accuses Clent of being enslaved to his schedule and his ioniser and his computer, completely forgetting about the human beings who work for him - and make him look good in the face of world opinion. Clent tells him of the stalemate situation and of the computer's recommendation. This changes things for Penley, but it doesn't change his opposition to Clent. Waiting is a sure path to destruction by the glacier. To Penley, the only chance is to activate and hope that the spaceship's reactor is safe. Clent refuses, still hoping to hear from the Doctor in time. It seems that the two men are not that far apart in their philosophies and yet they continue to spar, the conversation getting more and more heated.

Clent accuses Penley of being a coward, of running away in the face of their great challenge. Penley retorts that Clent is just a slave to the computer and that his freedom is more precious than anything else. Jamie joins in, trying to cut through this old tension to try and find someone to help his friends. A full-scale row erupts in the control room until Penley and Jamie are tranquillised by Walters and put out of action.

A livid Clent orders them both taken away. Jamie will be seen to, but to him Penley is nothing. A mere annoyance. Miss Garrett is forced to agree. Their trust must be in the world computer, despite the conundrum it faces. She says she is certain they cannot fail, but it sounds more like a hope than a fact.

From the cliff overlooking the base, Varga provides firing coordinates for Zondal back at the ship. The sonic cannon is being lined up with its target.

The Doctor and Victoria have been momentarily forgotten in the commotion and the Doctor has formulated a plan. He mimes to Victoria to start crying whilst he produces his phial of ammonium sulphite. Her histrionics don't initially rouse much response in the implacable Zondal but it does cover their conversation about their plan. He is hoping that his calculations are correct and that the chemical which is merely unpleasant to humans is lethal to the Martian warriors.

The sonic cannon is now in position and Zondal waits for the order to fire. Varga is to contact the base to make his demands.

On the Doctor's orders, Victoria distracts Zondal by telling him there is water seeping into the ship. The Doctor starts to attack the warrior only to find to his horror that the stopper on his phial is stuck! Zondal realises the ruse and is angered. He also sees the Doctor handing the phial to Victoria and demands that she hand it over.

The order to fire comes over the communicator just as Zondal starts to approach the Doctor. The loyal warrior turns from the inconsequential human and moves to the firing mechanism. Victoria gets the stopper out and tosses the chemical into Zondal's face.

Zondal staggers, gasping even more labouriously than usual, and starts to go down. Incredibly, he manages to stagger to the control panel as he falls, still intent on firing the gun. Varga's voice echoes through the communication system urgently ordering Zondal to fire.

The base begins to rock and shake. Sections of the ceiling fall in all around the frightened scientists. The records wing is entirely demolished and panic reigns.

Varga appears on the communications unit, ordering Clent to surrender or die. Clent refuses, all too aware of his duty to the world. Varga threatens to fire again and Clent knows he must play for time. He tells Varga that there is nothing to be gained by destroying the base - that they both have urgent needs and can help one another. He agrees to talk but does not surrender.

Varga agrees to talk but says there must be trust between them. Clent agrees there will be no traps or conditions and the communication link goes dead. Clent is not sure that Varga can be trusted, but they must go through with it. The base cannot withstand another blast from the warriors' weapon; they can only hope that talking can work.

Miss Garrett suggests bluffing the creatures using the ioniser. Clent does not see the merit in this, but an increasingly nervous Walters does. He doesn't just want to make threats though, he wants to act and he is becoming increasingly agitated.

Walters believes that Clent's insistence on sticking with the computer's recommendation has doomed them all to certain death. The machine is not taking into account the humans in the base, only its own preservation and the world programme. They are surrounded by danger - glaciers and homicidal aliens - and the computer can't do anything about that!

Becoming hysterical, Walters denounces the emotionless Clent and wishes for someone like Penley or the Doctor to help them, not the computer. In fact, the computer is the root of all their problems, he decides, and starts to attack it. Miss Garrett calmly shoots him with the tranquilliser gun and quiet returns to the control room.

The Doctor and Victoria have overheard all of Varga's communication with the base. He announces that he and his men are going in. Victoria thinks they should take advantage of this and escape now while Zondal is still unconscious but the Doctor wishes to take more concrete action. He begins to examine the controls of the sonic cannon, hoping to remove the threat the warriors hold over Clent.

Varga, Isbur, and Rintan stride into the ioniser control room, standing tall and imposing over the humans. Clent tries diplomacy, which fails. Varga made no promises and states bluntly that the humans will hand over what he requires.

While Clent tries to regain control of the "talks", Walters regains consciousness on the floor. He is a trained soldier and goes on the offensive immediately. He starts to roll forward with his tranquilliser gun and is promptly shot and killed by one of the warriors.

Varga takes this as proof of the humans' bad faith. He announces that he needs mercury isotopes for his ship's reactor. Clent tries to bluff Varga but the Doctor has already told him they are here. He knows Clent is lying and decides to shut the reactor down to extract the isotopes necessary. Clent and Miss Garrett protest, saying that the ioniser will be shut down as well as all light and heat in the base. However, the thought that the humans will perish only pleases Varga the more. He is ready to press on.

The Doctor overhears all of this over the ship's communications system. He knows he must work quickly to help the scientists. He tells Victoria he is going to try and turn the warriors' weapon against them. He finishes with the mechanism inside the ship and them moves outside to the gun barrel, Victoria urging him to hurry.

Clent tells Varga that they cannot simply turn off the reactor. It is too closely linked with the ioniser. If they don't power the system down gradually, the entire base could explode. There is no way to tell whether he is serious or not, but Varga believes this story. He orders the power to be run safely down by the technicians and urges them not to try and tricks.

However, Miss Garrett and Clent try to stall. Varga sees through this and tells them that he knows about their fear of his ship's engines. They will not use the "weapon" for fear of nuclear annihilation so they might as well shut it down. He points his sonic gun at Miss Garrett and orders her to work or die. Clent begs for her life but nearly gets himself killed in the process. In the end, Miss Garrett agrees to do as she is ordered and Clent is left alive. For now.

The Doctor works feverishly on the sonic gun. He explains to Victoria that he is trying to adjust the vibrations produced by the mechanism so that they affect the warriors without harming the humans in the base. The vibrations he's creating will affect mainly fluids, of which the alien warriors have more than humans. Also, their helmets will trap and amplify the soundwaves as well. As he finishes the modifications, he says he's confident it will work, although there may still be great danger for the scientists as well as Penley and Jamie.

In the base, Penley and Jamie are unconscious in the medi-control centre. Both men have been treated and then forgotten. Penley regains consciousness and stands up stiffly. He checks to make sure Jamie is all right and then heads out of the centre. Jamie is alive and well but dazed. He does not stir.

Shortly, the base's power has been run down to safety levels and Varga orders the reactor to be completely disconnected. Miss Garrett complies and the background hum of the ioniser dies out completely. Soon the glacier will surge forward and crush them all. Varga wants to move quickly.

However, Penley has arrived outside the control room and moves into action to stop the creatures. He has noted that they are perfectly at home in the ice age conditions that prevail and has formulated a plan. He adjusts the base's environmental controls, turning temperature and humidity up to full and the oxygen down to minimum. The effect is quick and dramatic. Varga and the other warriors stagger and gasp. The heat is too intense for them. Varga realises this is an attack and vows to destroy Clent and the others for this trickery.

At the same moment, the Doctor prepares to activate the sonic cannon. Victoria pleads with him not to, afraid that Jamie will be harmed, but the Doctor knows he must take the risk. He fires the gun.

An intense vibration shakes the base. Everyone, human and alien, puts his hands to his ears to shut out the noise but they cannot. The scientists collapse and the warriors stagger before the burst is shut down.

The Doctor contacts Varga on the communications unit. He demands that Varga retreat from the base or he will fire again. Privately, he tells Victoria that he dare not fire again for fear of doing serious damage to the humans.

Thankfully, Varga and his men retreat, but not without a vow to destroy the humans for what they have done. They do not notice as they go Penley lying unconscious on the floor, a victim of the sonic attack.

The Doctor knows that he and Victoria must not be in the ship when the warriors return. He fuses the circuits on the sonic cannon and races out with Victoria in tow. When the warriors return, they are angered to learn of the sabotage. Zondal regains consciousness.

The Doctor and Victoria reach the base without incident and find everyone inside unconscious. Victoria goes immediately to find Jamie and she is pleased to report that he is alive in the medi-control centre. The Doctor then sends her back to the TARDIS to wait. There is still some dangerous work for him to do.

Using the ammonium sulphite as smelling salts, the Doctor revives Penley first. He is stunned at the damage that he sees. The Doctor spares no time to explain. There is work to do.

In the spaceship, Zondal stands before his commander, awaiting punishment for his failure. However, Varga will not punish him just yet. There is no time. He knows that the scientists will now activate the ioniser at full power and the ice will melt. They must be ready to break free from the ice as soon as they are able.

The Doctor and Penley wake up the rest of the ioniser staff. Jamie, too, is now up and around, his paralysis cured. The Doctor orders Miss Garrett to reactivate the ioniser, but she will not do so until Clent tells her to. The Doctor tells Clent of the ion reactor in the spaceship and Clent despairs. They dare not use the ioniser then due to the chance that the ship's engines will explode.

Penley and the Doctor are stunned at this. There is an equal chance that the engines won't explode. Both men believe that the risk is worth taking in order to defeat the glaciers. Clent disagrees, sticking with the computer's original recommendation: do nothing. The Doctor tries to remind Clent that the computer is thinking of itself. Either course of action will destroy it and so will recommend nothing. Penley states the only option: Clent must override computer control and make a decision himself!

Clent resists, horrified at the prospect. The world programme will be completely disrupted if he decides incorrectly. But Penley spells out to him that their situation is unique. No other base has a glacier so close. No other base has homicidal alien warriors bearing down on them. The computer is not taking into account the true situation. Only a human mind can do that. Clent must act without the computer. There is no other choice.

Amazingly, both Clent and Miss Garrett refuse to do this. So long have they depended upon the computer that they do not trust themselves to make the right decision. The Doctor tries to convince Penley to act but Clent asserts his authority. He demands to make one more appeal to the computer.

Penley agrees and steps back. Clent feeds in the final piece of information the computer has been waiting for: the fact that the alien ship contains an ion reactor. Can they use the ioniser or is there another solution?

Amazingly, the computer begins to sputter and spew gibberish. Its console spins round on its own and is soon winding down. The computer has gone mad! Clent and Miss Garrett are distraught. They are bereft of help.

Penley steps up to the challenge and makes the decision. They must use the ioniser at full power to turn back the glacier. Whatever happens to the warriors' craft will happen. At least the glaciers will be contained.

In their spaceship, the warriors detect the swift melting of the glacier. Their ship will soon be free. At first they despair that their engines do not have enough fuel for launch but a check soon shows that that is not the case. A powerful hum reverberates through the ship and Varga is triumphal. He orders Zondal to carefully increase power. They must time their take-off boost perfectly once they are free of the ice.

The ioniser is nearly at full temperature. The instruments Arden placed in the ice record the activity around them. They can resist all but the highest temperatures. When they cease registering, then the ioniser is at full power. The mighty hum in the base rises. Penley is at the controls but Clent stands by, nearly hysterical with panic. He is certain that Penley has made the wrong decision and that they will all be killed. Penley accepts the risk and powers the machinery to full.

On board the spaceship, Varga has made a grim discovery. What they thought was residual power was only heat build-up from the ioniser. They have no way to escape the glacier, the floods, or the intense heat of the ioniser. The ship shakes, increasingly violent, and is now full of smoke. The instruments explode one by one. The heat build up causes the warriors to stagger and they are soon consumed in the molten heat of the ioniser.

At the base, everyone is breathless as they wait for the alien ship to explode. All eyes watch the seismic monitors. At last they register, a small shockwave only. There has been a minor explosion in the glacier. It is the alien spacecraft being destroyed, in a small localised explosion, not a nuclear detonation. They have won and the risk has been proven worthwhile.

The Doctor and the others heave sighs of relief as they realise that they are safe. Penley's gamble has paid off. He orders all circuits switched to automatic and then tied in to the world ioniser control. They are back with the world programme and the glaciers will be contained.

Penley turns to Clent. At first the leader seems lost, but his relief and admiration come through. He calls the maverick Penley the most insufferably irritating and infuriating person it has ever been his privilege to work with. They share a smile. Clent has learned much.

Clent chides Penley about his poor report writing skills and how he prides himself that he's always written his own reports and speeches. Without a computer. Penley laughs at this jibe and starts to ask if Clent will include the mysterious Doctor in his report. However, there is no sign of the Doctor in the control room. Jamie is gone as well. They have vanished.