A blue-uniformed man stumbles across a bleak foggy landscape. He is struck on the right side of his torso by an arrow; he groans, and tumbles down a rocky slope into water.

'Oh I do like to be beside the seaside, I do like to be beside the sea,' sings the Doctor cheerfully in the TARDIS control room, as he twirls a multi-coloured beach umbrella over his head. Sarah, scantily-clad for the beach, is checking her holiday gear - sunglasses, sun lotion, water wings - but the Doctor assures her that she will not need water wings, because on Florana the water is effervescent and the bubbles support a swimmer. He remarks that he always comes back from Florana feeling a hundred years younger.

Sarah notices a red light flashing on the TARDIS control console, and the central column comes to a stop. The Doctor hypothesises that they are experiencing a mains power failure; then a violent explosion of sparks on the console causes Sarah to cry out. The control room darkens as the lights fail. The Doctor cuts in the TARDIS's emergency units, and the lighting is restored. But a few moments later the lights start failing again, and the Doctor and Sarah are thrown to the floor as the TARDIS lands. With a distinctly unhealthy version of its usual landing sound, it materialises on the dark and foggy landscape.

The Doctor and Sarah get to their feet. Eerie groaning sounds can be heard in the control room. 'It's as if the TARDIS was dying,' exclaims Sarah. There is sufficient charge to operate the scanner briefly, but its image of the external environment shows only fog, and quickly fades. Perhaps the fog is what has put the TARDIS out of action, the Doctor suggests. With a whine and a groan the remaining lighting in the control room fades out. Sarah has the idea of using the emergency storage cells, but after a moment's functioning these fail too. The Doctor points out the complete silence in the TARDIS, despite the fact that 'the TARDIS is a living thing - thousands of instruments. Its energy sources never stop.' 'Well they have now,' Sarah observes unhappily. A battery-powered torch from the Doctor's cabinet quickly fails, just like the other lighting. Only an oil lamp, which the Doctor pulls from beneath the console, proves immune to this mysterious effect.

The Doctor wants to go out and see where they are. He ignores Sarah's protest that they should be on Florana. The TARDIS door is normally power-operated; the Doctor has to open it manually with a crank, while Sarah holds the oil lamp so that he can see what he is doing. Fog creeps into the control room as the door opens. The Doctor takes the oil lamp and leads the way out into a cold, dark, uninviting, boulder-strewn environment. Sarah makes an ironic remark to the effect that this is obviously not Florana. This makes the Doctor irritable, but his rebuke is cut short when Sarah screams at the sight of a mass of rock resembling a looming figure. The Doctor identifies it as 'some sort of life-form that has become petrified'. He doubts if anything has grown here for centuries, and observes that if the rest of the planet is the same as this spot, life on it certainly doesn't depend on photosynthesis. 'What life?' Sarah protests. 'Some power emanating from this planet has stopped the TARDIS's energy banks,' explains the Doctor. 'Now logically that power can hardly be of geological origin.' A frightened Sarah wants to 'fix the TARDIS and clear out,' but the Doctor points out that they cannot do this without discovering the source of the power that is keeping them here. Meanwhile, a dark figure observes them surreptitiously from behind a rock.

Sarah is aghast at the prospect of being permanently stranded. The Doctor proposes that they begin by checking the immediate area. Sarah heads for the TARDIS to put on some warmer clothes, having anxiously checked with the Doctor that he will not go away. But the Doctor apparently cannot resist a little stroll around, his oil lamp held in front of him; and almost at once, two menacing cloaked figures move after him.

After a short time, while the Doctor is kneeling to examine the planet's surface with his lamp, one of the figures rushes at him, and is upon him too quickly for him to defend himself.

Sarah, now more suitably dressed for this environment, leaves the darkened TARDIS and discovers to her alarm that the Doctor is gone. She races through the foggy wilderness to the place where he was attacked. She finds the abandoned oil lamp, still burning and - to her horror and disgust - with blood smeared on it. She drops the lamp and runs in panic back to the TARDIS.

She breathlessly enters the control room, but, unseen by her, a figure clad in a ragged brown cloak has already got inside. Straining with the crank, she closes the door. The cloaked figure creeps up behind her; she turns just in time, screams, and beats the intruder to the floor with five blows of the crank. Now she has to get the door open again. On the floor, the intruder's left hand twitches; he crawls forwards and grabs Sarah's leg. She screams and hits the intruder with the crank again - then runs from the TARDIS, observed by other cloaked figures.

It seems to be growing lighter. The Doctor, his hands tied, is led along by the two cloaked figures, one walking in front of him and the other behind. He falls forward; the figure behind moves up to him, and the Doctor, rising, pushes him violently to the ground. He wrests the other captor's flaming torch from him, turns and hits the first captor with it. He turns again, and manages to untie his own hands. He stumbles backwards as the cloaked figure advances, but succeeds in fending him off, hits him, grabs his arm, drags him to the ground and finishes him off with a martial arts-type blow. The Doctor looks quickly around and makes good his escape.

Elsewhere, more cloaked figures are on the move. Sarah has to evade them, dodging between upright rocks - when to her astonishment, she beholds an enormous pristine-white temple-like complex rising up from the barren surface. It is surmounted by a tower, with a light that flashes rhythmically, accompanied by an eerie funereal tone. Fascinated, she moves towards it.

It is daylight, and the Doctor walks alone in the dry, barren wilderness, observed from above by another cloaked creature. He spots a trip-wire almost concealed in the sand. He triggers it deliberately by throwing a stone onto it, and a huge boulder is dislodged from a nearby rise and thunders down towards him as he quickly takes refuge. A blue-uniformed man, wielding a knife, hurries down and grapples with the Doctor; but a second blue-uniformed man appears and stops the fight with an angry yell of 'Galloway!' The newcomer apologises to the Doctor, but Galloway has spotted something moving; the newcomer proposes they get back to the base, where they can talk. All three move off, silently observed as ever.

The base is a metallic structure incongruously embedded into the sandy-coloured rock. On arrival, the second uniformed man has to prevent a third - a young man called Peter - from shooting the Doctor with a bow and arrow. He tells Peter to keep a sharp watch, because of the movements heard by Dan (Galloway).

Inside, the base has a makeshift appearance. Another member of the team, an attractive brown-haired young woman, asks the second man if they have found Jack: 'I'm afraid not,' he replies. He enquires after the health of another man who lies unconscious in the room, his head bandaged. 'He's not good,' she says. 'And we're down to our last pack of sulfagen tablets, Richard.' The woman explains to the Doctor that 'the Exxilons took us by surprise, right after we landed.' Galloway, who has a Scottish accent, remarks that the Exxilons' stone knives have some sort of poison on them: 'They're dirty wee fighters,' he adds.

'Richard' now introduces the team to the Doctor properly: 'I'm Captain Railton, second in command of the expedition, and' (indicating the unconscious man) 'this is Commander Stewart. And that's Jill Tarrant, our civilian geologist.' Jill asks if the Doctor is from Earth, and he replies, 'In a roundabout sort of way, yes.' 'Lieutenant Galloway is our weapons officer,' Railton says; the Doctor good-naturedly reproves Galloway for his earlier ill-mannered way of introducing himself. 'The chap outside is Lieutenant Peter Hamilton,' concludes Railton.

The Doctor enquires if this is a military expedition. 'MSC,' says Railton. 'Marine Space Corps.' Galloway wants some answers from the Doctor about his movements on the planet; the Doctor says he was heading back to the TARDIS ('that's my spaceship'). He says he hopes Sarah had the good sense to stay inside it, but remarks that she is 'rather headstrong'. Railton says that Sarah should be all right as long as she is careful; 'the Exxilons seem to be night creatures.' 'Just so long as she didn't go near that forbidden city of theirs,' says Jill. Railton explains that only the Exxilons' high priests can visit the city; Galloway adds that other intruders are killed.

Railton shows the Doctor photographs of the city from the visual file: 'It's a fantastic building. Must have been constructed thousands of years ago.' Jill adds: 'And not by a stone-age tribe like the Exxilons, either.' Railton continues: 'They treat it as a sort of shrine. If you're caught near it, it's certain death.'

Meanwhile, Sarah is just arriving at the outermost city wall, which is patterned with peculiar symbols. When she touches the wall, that section of wall lights up with a bizarre humming sound. The symbols, too, light up when touched. All the time, the rhythmic chiming that accompanies the city's main beacon can be heard. 'No doors...no windows,' Sarah muses to herself. Suddenly, a cloaked Exxilon ambushes her from behind.

At the base, the Doctor and the MSC team are sharing a meal. Railton explains: 'As soon as we got within range of Exxilon, we had total malfunction on all instruments. We managed to touch down without damaging the ship, but we can't take off again until we find some way of restoring power.'

Jill explains that the purpose of the expedition is to collect parrinium: 'It's a chemical. It can be found in minute quantities on Earth, but it's so rare there that it's virtually priceless. A chemical-detecting satellite did a fly-past on this planet, and registered that it was as common here as salt.' Galloway is astonished that the Doctor does not know what the humans want the parrinium for. 'The outer worlds are being ravaged by a disease,' Jill tells the Doctor. 'The colonists are dying in their thousands. Another ten million men, women and children will die unless we help them - and help them quickly. Every hour we're stuck here on this planet, the death toll's mounting.' Parrinium, explains Railton, both cures the disease and immunises against it; but a lot of the chemical is needed quickly. 'If it's not delivered within a month, it'll be too late,' observes Jill.

Railton says that before they had total power failure, the team managed to get a message off to Earth, requesting a relief ship. But Galloway thinks that if the message had been received, help would have come by now. 'I think we can safely assume that no help is coming,' the Doctor says. He is about to make a suggestion to Railton, when Hamilton bursts in and yells that the relief ship is here. Commander Stewart stirs weakly on his bed; the other four MSC members and the Doctor rush outside to see.

Hamilton says he heard the ship, up in the haze, over to the north a little. Galloway says it is probably making a spiral descent. 'Here she comes!' cries Jill, and they see a bulky-looking silver flying saucer coming speedily in to land. Railton sees that it will land in the next valley, and they hurry off to meet it. Behind them, two Exxilons emerge from hiding. Inside the base, Commander Stewart looks up in fear...

The Exxilons have Sarah tied up and behind bars, in a chamber lit by candles and flaming torches. A party of the cloaked creatures advances menacingly towards her cage, their alien religious chants filling the smoky air. A booming voice accuses her: 'You have looked upon the sacred place. You have walked where only the chosen may go. You have defiled the place of all power.' Two Exxilons - now clearly seen as bald brown-skinned humanoids with large dark glassy eyes - enter Sarah's cage, untie her bonds and lead her out. His hands raised in ceremonial gesture, the High Priest booms: 'A judgement has been made. Prepare her for sacrifice!' The other Exxilons' chanting moves up to a higher pitch of excitement. Sarah struggles desperately in the grip of her captors.

The saucer settles to the ground as Jill, Railton, Galloway, Hamilton and the Doctor arrive to meet it. Hamilton is concerned that it does not look like an Earth ship; Galloway suggests it may be 'that new Z47 they've been planning'; Railton notes that it is not a Space Corps craft; and Galloway hopes that the ship has not run into the same power block that his team did.

Eyes closed, the Doctor holds grimly on to a terrified Jill as they wait for death. But the Dalek guns are not working - the three-pronged tips of their weapons flutter impotently, but no death-rays emerge. 'Maximum power!' orders the lead Dalek. 'Fire! Fire! Fire!' But still nothing - there is a total power failure in the Daleks' armament circuits.

'Well well well,' exclaims the Doctor delightedly. 'Daleks without the power to kill. How does it feel?' The lead Dalek's voice sounds frightened as it warns him to keep back. The Doctor is confident now, realising that the Daleks' weapons have been affected by the same energy blackout that has stranded the TARDIS and the MSC team. The Dalek assures him that 'superior Dalek technology will overcome interference shortly - meanwhile you will obey our orders'. But the Doctor reminds the Dalek that it is in no position to give orders - all those present are equal, and all powerless. The Dalek's rant in reply is cut short by Railton, who recommends that the humans and the Daleks join forces. 'Daleks do not require the cooperation of inferior creatures' is the predictable response. But Railton points out that there are only four Daleks and five humans, on a planet swarming with aggressive creatures: 'That's a fact that must penetrate even Dalek arrogance,' he adds boldly. The Daleks withdraw to discuss Railton's proposal. The Doctor warns Railton not to trust the Daleks. Galloway simply feels that the humans don't need the Daleks' help, but here the Doctor disagrees: 'They're brilliant technicians. It was their inventive genius that made them one of the greatest powers in the Universe, remember that.' Hamilton hates the thought of working with the Daleks because his father was killed in the last Dalek War; but Railton points out that he was just one man, and 'ten million will die if we don't get off the planet with the medical supplies.'

In the smoky chamber of the Exxilons, Sarah is surrounded by the chanting, cloaked creatures. An Exxilon advances upon her, bearing a flaming torch.

Hamilton and Galloway privately discuss the idea of cooperating with the Daleks. Hamilton is sceptical about Railton's judgement; and Galloway remarks that Railton is 'scared of the wee salt-shakers'. The Doctor returns with news of the Daleks' decision: they are willing to cooperate, in the absence of any other choice. But he repeats his warning to the captain not to trust them an inch. The Daleks have told the Doctor that several of the planets they have colonised are suffering from the same disease that has attacked the humans, and that the Daleks are dying in their millions - the Daleks need the chemical parrinium just as badly as the humans do. For this reason, the Doctor thinks the Daleks will cooperate.

But a nearby conference among the four Daleks confirms the Doctor's mistrust of them: 'We will cooperate until they are no further use to us.' 'And the true reason that we need the chemical parrinium must remain secret...they must continue to believe there are only four of us.'

The Daleks return to declare a truce - until power is restored. Railton recommends that they should all go to the mining dome, which the humans have set up near the stratum of parrinium they have found. Railton suggests that perhaps the Daleks can suggest a quicker technique for refining the ore, which is slow work without power. They set off, Railton leading the way.

Meanwhile, in the dome, wreckage is scattered around; and Commander Stewart's bed is empty.

The party consisting of Railton, Galloway, Hamilton, Jill, the Doctor and the Daleks is en route for the mining dome, observed from higher ground. The Doctor cautiously scans the barren landscape around them. He turns his head as some dislodged sand rattles down a nearby rise. The Daleks come to a halt; the rest of the party stops with them, aware that something is amiss.

Suddenly Railton, standing a little apart from the others, is struck in the middle of the back by an arrow, staggers forward and falls to the ground. The Doctor urges everyone to take cover; the lead Dalek hysterically orders the others to locate the enemy. Jill runs to Railton's aid. Exxilons gather on the rise overlooking the group. The Doctor runs with Galloway and Hamilton; at Jill's request for help he turns round and runs to join her at Railton's side; but the captain is beyond help. Jill still does not want to leave Railton, but arrows are thudding nearby, and the Doctor pulls her away. They join Galloway and Hamilton behind the shallow cover of a sandy ridge. Arrows hit the ridge. The Doctor suggests that they might make a break for it if the group of Exxilons is small. Galloway points out that as next in seniority to the captain, he is now in command, but he has no alternative suggestion. More Exxilons are gathering above, some with bows and arrows, some with spears - it is a large group of Exxilons after all.

Galloway aims his own bow and shoots one of the Exxilons, exciting the rest into heightened agitation. More Exxilon arrows are fired, and spears are thrown. One of the Daleks instinctively yells 'Exterminate!' but of course its gun still does not work. Realising its helplessness, the grunting Exxilons hurry down and surround the Dalek, battering against its metal shell with their bows. They withdraw from the hysterical creature, and its head explodes, leaving the headless shell on fire. The Doctor and the three humans watch from cover as the Exxilons caper triumphantly around the blazing wreckage of the Dalek.

A second group of Exxilons now approaches, with Commander Stewart captive. Clearly weak, he falls to the ground, and urges the others to give themselves up. The Doctor agrees with Hamilton and Jill that they should follow Stewart's instruction, 'when the only alternative to living is dying.' They emerge from cover, the Doctor and Jill raising their hands in surrender. Hamilton follows, then finally Galloway, reluctantly.

At the entrance to the Dalek ship, the returning Dalek announces the capture of the patrol 'by alien beings' and the destruction of their comrade. 'We will report,' replies another Dalek, and they go inside the ship.

Sarah's head is held fast by an Exxilon as a large goblet of smoking incense is waved in front of her face. She passes into semi-consciousness, and the incense is taken away. Only the pressing throng of Exxilons prevents her from slumping to the floor. The Doctor and the humans are led captive into the chamber. The Doctor is horrified to see Sarah there, and demands to know what the Exxilons are doing with her. He is told that Sarah 'has defiled the place of our gods - she is to be sacrificed.' After a moment pondering this, the Doctor elbows both the Exxilons behind him and rushes forward into the main group of Exxilons. There is a scuffle, but the Doctor is swiftly overwhelmed by the throng of Exxilons; he is crushed to the floor and loses consciousness...

Sarah emerges from her incense-induced trance, and meets Jill, who is tending the Doctor. The Doctor is also coming round. 'Neither of you is very popular,' Jill tells Sarah anxiously. 'The Doctor attacked their High Priest. From what we can gather, that's about the worst crime you can commit on this planet.'

Nearby, Hamilton and Galloway are with the unconscious Commander Stewart. Galloway is sure that the Doctor's 'crazy' behaviour has ruined their chance of a deal with the Exxilons. He doesn't care that Sarah would have been killed without the Doctor's intervention: 'We have no loyalties to them, they're not part of our mission.' All Galloway is interested in is getting medical relief to the millions who need it: 'If some people we don't even know have to die in the process, well that's just too bad.' Two Daleks appear and tell Hamilton and Galloway that they have decided to offer the Exxilons Dalek knowledge and technology in return for their assistance. But when Hamilton mentions the Doctor and Sarah, the response is: 'The Doctor is an old enemy of the Daleks. The girl is unimportant.' The lead Dalek tells an Exxilon guard that it wishes to speak with the High Priest.

Jill tells Sarah and the recovered Doctor that she doesn't know what the Daleks are up to, but that she doesn't like it. The Doctor warns Jill that it might be better if she did not associate herself too much with him and Sarah: 'I think we're the flies in a jar of very nasty ointment.' He tells Sarah regretfully that her sacrifice has merely been postponed, and that 'in the next performance' both of them will die.

The lead Dalek is told that he has been granted an audience with the High Priest; but only that one Dalek is allowed to leave the cell.

Jill optimistically suggests that the Daleks may be trying to negotiate a way for all the captives to be freed; but with a little shake of his head, the Doctor dismisses this as pure wishful thinking: 'The Daleks won't give us any loyalty. And I don't expect too much help from your space chums either.'

Inside the Dalek ship, the Daleks are testing improvised substitute weapons - mounted as normal, but firing conventional bullets - using a small model of the TARDIS for target practice. They glide out of the ship and gun down two Exxilons who were waiting outside with bows and arrows. The Exxilons scream horribly as they die. 'Primitive weapons moderately efficient,' announces one of the Daleks.

Two Exxilons arrive to take the Doctor and Sarah from the cell. Hamilton is desperate to help, but Galloway urges him to stay focused on the mission. The lead Dalek enters the cell and says that it has been agreed that the Exxilons will discuss terms - once the sacrifices have been completed. The other captives are to follow and observe. The Dalek turns and leads Hamilton, Jill and a second Dalek from the cell; Galloway, on the point of following, is called back by the dying Commander Stewart. 'You're not fit to command,' Stewart gasps. 'I'm doing what's necessary, sir,' he responds. 'I'll get that chemical to the peoples of the outer worlds whatever it costs.' Stewart accuses Galloway of wanting to achieve this for his own glory, rather than for anyone else's welfare. As his last order, Stewart tells Galloway that he is appointing Hamilton over him. Galloway looks terribly hurt. 'Peter will take command,' gasps Stewart as he dies. Galloway gently closes Stewart's eyes. 'Sorry, Commander. Couldn't quite hear what you said.'

Flames flicker and the smoke of incense fills the air; it is time for the Doctor and Sarah to be sacrificed. Cloaked Exxilons mill around; the other captives observe grimly. A goblet of incense is waved in front of the condemned travellers' faces. Suddenly the sharp snap of gunfire interrupts the ceremony. An improvised Dalek gun fires upon the assembled Exxilons. The Doctor and Sarah make a run for it; so do Galloway, Jill and Hamilton. It is a massacre. In the smoke and confusion, the Doctor and Sarah climb up a shallow ledge, and then jump down and out of the chamber, into the tunnel beyond it.

'The Daleks don't seem to be following us,' the Doctor tells Sarah. 'Those robot things,' says Sarah. 'Are they locals?' 'Hardly,' the Doctor replies. 'They're probably the most technically advanced and ruthless life form in the galaxy.' 'But if they're robots,' asks Sarah, 'how is it their power's not affected? They can't half move!' 'Because they're only half robots, Sarah. Inside each of those shells is a living, bubbling lump of hate.' 'You mean they've got legs?' 'No, they move by psychokinetic power.' 'I see.' 'Do you?' 'No,' Sarah admits miserably.

Back in the chamber, the Exxilons have stopped trying to resist the Daleks. The lead Dalek orders a second Dalek to bring the High Priest to him. Galloway asks what the Daleks want with the High Priest. He is told that the Daleks need the Exxilons' cooperation, but that now the cooperation will be on the Daleks' terms. If the Exxilons do not agree to this, groups of hostages will be selected for extermination until they do. Galloway seems to approve of this scheme. He suggests that the Exxilons should be made to supply working parties to mine the parrinium, but the lead Dalek does not want his advice - the Daleks are in command, and Galloway is to obey their orders.

In the tunnel, Sarah asks the Doctor how the Exxilons intended to sacrifice them just by dumping them down here. The Doctor does have an idea about this, but says it isn't one of his favourites: 'I think they expect that sacrifice to be completed by something else - something that lives down this tunnel.' They hear a distant howling roar. Fighting down her nerves, Sarah tells the Doctor to keep his next idea to himself. She smiles shakily at him. He puts an arm reassuringly round her shoulder, and they move forwards into the gloom.

In the Exxilon chamber, Galloway tells Jill and Hamilton about the 'deal' that has been struck with the Exxilons: they will provide a working force to help mine the parrinium; the humans will be in charge of this, along with two Daleks, while the other Daleks investigate the cause of the electrical interference; and in return the Exxilons will get help wiping out their one enemy, a sort of breakaway group. Hamilton and Jill are horrified at this proposal. But Galloway, while obviously not comfortable with the arrangement, still feels it is worth it: 'The creatures on this planet are primitives, they don't count.' Reluctantly he tells them the final part of the deal: the Doctor and Sarah have to be brought back, dead or alive. Jill looks horrified, and Hamilton says Galloway is totally ruthless. 'The Daleks have gone down the tunnel after them,' Galloway tells them. 'Just in case the creatures that live down there don't get them first.'

In the tunnel, the Doctor and Sarah hear the howling roar again, closer this time. The Doctor suggests it is probably 'some sort of subterranean wind effect.' Another roar rings out. 'Who are you kidding?' Sarah asks. 'Myself, chiefly,' the Doctor replies.

Somewhere behind them in the tunnel, two Daleks are following.

The Doctor and Sarah continue along the tunnel. A humanoid shape, ribbons of light glinting on its surface, gropes its way out from the rocks they have just passed.

Candles illuminate the tunnels. The roaring is heard again. 'I just heard your wind effect gnashing its teeth,' Sarah says nervously. They have arrived at a junction. The Doctor wants to make a short reconnaissance by himself down one of the available tunnels, while Sarah stays behind to keep watch - she has a choice of three different ways to run if anything nasty should happen. The Doctor will check approximately the first half-mile of the tunnel to see if it holds any promise; if so, he will come right back. Sarah wishes him luck. The Doctor heads off along the tunnel, leaving Sarah glancing nervously around in the gloomy, candle-lit cavern. Unseen by Sarah at first, something alien is creeping into the cavern to join her...

Not far away, the two Daleks are approaching.

Sarah, sensing a presence, asks nervously if somebody is there. She turns, paces a short way, stops, and waits. Then suddenly, seemingly filled with conviction that something is behind her, she turns - and gives a strangled cry of horror. A smaller version of the surface-dwelling Exxilons, its grey parchment-like skin giving it an almost corpse-like appearance, is leaning into the cavern, staring at her with its enormous glassy dark-grey eyes, and groping in the air towards her with its right arm.

The machine-creature swings its head at the Doctor. As he dodges out of the way, it strikes the tunnel wall next to him, which ignites in a violent burst of white flame. He falls backwards to the tunnel floor. The monster swoops down on him, and he rolls out of the way just in time; the tunnel floor ignites explosively where the creature's head touches it. The snake-head bobs menacingly in the air, poised for a third strike.

Sarah, utterly terrified, warns the diminutive corpse-like creature to keep away from her as it enters the cavern. 'Please...' implores the creature, in a voice like that of a frail old man, sounding almost as scared as Sarah. 'Who are you?' she cries. 'I am called Bellal,' says the creature. 'I mean you no harm. I will help you if I can. I do not share the beliefs of those who would have killed you - they consider that I and others like me are their enemies.' 'And are you?' 'Oh no - we seek merely to save the Exxilon race from extinction. We do not share their beliefs, and for that reason we are persecuted and driven to live in the secret places beneath the surface.' Bellal gestures around the cavern, seemingly terribly anxious to make himself clear. Sarah, still terrified, asks how many of Bellal's group there are. 'Very few against so many,' he replies; then adds agitatedly, while Sarah desperately tries to avoid touching him: 'I will answer all your questions, but first we must go from here. We have a place that is safe. Follow me and I will take you to it.' But Sarah says that she cannot go with Bellal, because she is waiting for the Doctor. When she indicates which path the Doctor took, the little creature gasps in horror. 'That way leads to death!' 'Well you've got to help him!' urges Sarah, but Bellal shakes his head vigorously in fear. A distant roar is heard. 'It may already be too late,' he says. 'But we will do what can be done.'

A second creature, of similar appearance to Bellal, hurries into the cavern with news that two of the 'creatures from the spacecraft' are coming this way. 'Machine creatures?' asks Bellal, and the newcomer nods. 'Daleks,' Sarah confirms. She and the two natives hide behind a large rock as the two Daleks glide into the cavern, eyestalks swivelling as they look around. 'We will search independently,' announces one. 'Fugitives are to be exterminated on sight.' Each Dalek heads off down a different tunnel; one of the tunnels is the Doctor's. Sarah is desperate to warn the Doctor, but as she starts to hurry towards his tunnel, the two little Exxilons restrain her. 'It will be no use!' Bellal insists to the struggling Sarah. 'You must stay silent or you will destroy us all. You must understand - it is beyond our power to help him now.' Sarah turns her head and gazes helplessly at the entrance to the Doctor's tunnel.

Cautiously, the Doctor creeps sideways along the tunnel; at present he is succeeding in evading the snake-like machine-creature, which lurks at a junction behind him. Now the Dalek which followed the Doctor arrives, and the two monsters confront each other, while the Doctor stays out of sight. The Dalek peppers the snake-creature with bullets from its improvised gun, but without effect. Instead, the snake-creature dips its head to touch the Dalek's, which bursts into flame. The flames quickly die away, and the Dalek keeps firing; but a second touch from the snake-head, and then a third, fourth and fifth, gradually wreck the Dalek while the Doctor watches delightedly: 'Oh, good shot sir! A hit! Yes, a palpable hit!'

In the cavern, Bellal urges Sarah not to go after the Doctor, because the other Dalek will have heard the noise of the battle and will be on its way back. Sarah fears that the Doctor may be hurt. 'We must hurry!' urges Bellal's friend. Bellal says that he will have some of his people look for the Doctor as soon as it is safe to do so; then he hears something coming from the Doctor's tunnel, and Sarah and the two natives hide again. Mesmerised, Sarah watches from cover as a Dalek gun-stick creeps into view from the tunnel entrance...but behind it is not its original owner, but the Doctor, who was cautiously holding the gun-stick out in front of him. Overcome with relief, Sarah runs to the Doctor and embraces him: 'You're safe!' She is anxious to hear what happened, but the Doctor calms her down. 'I've just had a face-to-face confrontation with a rather nasty root,' he explains. 'It obviously wasn't very fond of me, but it positively hated the Dalek.' The Doctor surmises that it was one of the city's underground support systems. And the result of the battle in the tunnel: 'The root won. Dalek nil.' The gun-stick is about the only part of the Dalek to have survived undamaged.

Bellal hurries forward, and the Doctor sees the two cave-dwelling Exxilons for the first time. He raises the Dalek gun-stick defensively, but Sarah stops him and assures him that they are friends. 'The other Dalek is coming this way!' Bellal announces. The two Exxilons hurry off, and Sarah goes after them. The Doctor pulls her back by the arm: 'Who are your shining friends?' he asks, as if not sure whether to trust them yet; but she tells him there will be time for explanations later, and urges him to follow. A moment after their departure, a Dalek glides into the cavern, and then out again, along the same tunnel the Doctor's party left by.

The four companions run along the tunnel, the two Exxilons leading the way. Bellal urges the others to hurry up, as the Dalek is coming this way. Sarah thinks they have arrived at a dead end; but there is a narrow crevice they can squeeze through, which will widen out after a little while. The Doctor sends Sarah through first and then follows, vowing to go on a diet one of these days. The Exxilons enter the gap last. The Dalek arrives with five shrieks of 'Exterminate!' and fires its gun into the crevice; but there is no straight line to a target, and the Dalek cannot follow. It gives up and turns away.

The Doctor asks if anyone is hurt; it seems not. He now wants to get back above ground so that they can set about restoring power to the spaceships, and helping the mission from Earth. Sarah observes that the Earth mission seems 'pretty pally' with the Daleks, but the Doctor knows that the humans will be wiped out without a qualm as soon as they cease to be useful to the Daleks. Bellal says that his people are watching what is happening above ground and will report immediately what happens. The four set off again.

Up above it is broad daylight on the desolate planet, and one of Bellal's people is secretly observing, from above, the progress of the mining party. Surface-dwelling Exxilons work around a shallow pool with primitive tools and pan the rocks for parrinium, while a Dalek supervises the activity and Galloway strolls about. The Dalek complains to Galloway that the Exxilons are working too slowly; Galloway blames this on the primitive equipment they are using. The Dalek says that the work force must be increased; but Galloway remarks that the High Priest was not very cooperative, and that they were lucky to get the workers they have. The Dalek is not interested, and glides away demanding: 'More workers! More workers! More! Exxilons will obey our commands.' 'And if they don't?' asks Galloway. 'Their leaders will be exterminated.' 'But - we made an agreement,' Galloway protests. 'It was expedient,' is the Dalek's reply. 'You will advise the High Priest that his people will comply with Dalek law or suffer the consequences.' Galloway protests again but is simply told, five times, that he will obey. He wearily returns to the job of urging the Exxilons on, rebuking them for their idleness.

In the Dalek ship, a Dalek announces: 'This planet is now under Dalek control. Our commands will be obeyed. Those who resist will be exterminated.'

As they walk along a candle-lit underground tunnel, Bellal explains to the Doctor: 'Exxilon had grown old before life had even begun on other planets. Our ancestors solved the mysteries of science, built craft that travelled through space. They were the supreme beings of the Universe.' 'What destroyed their power, war?' asks the Doctor. 'No, they created their own destruction. Using all their knowledge, they built a City that would last through all of time...they used their sciences to make the City into a living thing. It could protect itself, repair itself, maintain itself. They even gave it a brain.' 'I see,' says the Doctor. 'So the City became an entity. Greater and more powerful than the many small parts that had created it.' 'Yes!' confirms Bellal. 'It then had no need of those who had made it. Our people had created a monster. They tried to destroy it. Instead it destroyed them and drove out the survivors. Now we, and the other Exxilons you met, are all that remain.' 'What separates you from the others?' Sarah asks. 'They have made the City their god. They worship and fear it. They even make sacrifices to it.' This part of Bellal's account has special resonance for Sarah. The Doctor asks Bellal why his people, in contrast to the surface-dwellers, do not fear the City. 'Yes, we do fear it, but we don't worship it. Our aim is to destroy it. Unless we succeed, our race will vanish from this planet.' After a sombre pause, the Doctor says: 'Yes. Well, I think the time has come to do something about this City.' He moves purposefully forward.

At the diggings, the Exxilons are still at work, some of them waist-deep in the dirty brown water of the pool, while three Daleks supervise. Hamilton and Jill are also supervising the progress of the work. The three Daleks come together to discuss their plans: explosive charges are to be placed on the City's beacon, while a patrol of Daleks enters the City to investigate scientific installations. The Daleks' assumption is that the beacon is the source of the interference which has stranded them on the planet, and that destroying the beacon will restore electrical energy.

Suddenly a snake-like machine-creature, like the one that confronted the Doctor underground, starts to emerge from beneath the surface of the water. The Daleks, Jill and Hamilton, and the Exxilons notice it almost simultaneously. Hamilton shouts a warning to the Exxilons in the water, who scramble for safety as the creature rears higher and higher above the surface of the pool. The huge creature swoops and makes contact with an Exxilon in the water; his cloak ignites in a blaze of orange fire, and he collapses into the water. The Exxilons look on, awestruck, as the creature's head hovers, poised for another strike. There is a chatter of bullets from a Dalek standing on a rocky ledge high above the pool. The snake-head swings round and strikes at the Dalek; the Dalek's head crumples in a burst of flame and smoke, and with despairing cries it tumbles down the steep slope into the water. The Exxilon onlookers grunt in superstitious awe; the other two Daleks simply look subdued, and do not try to attack the machine-creature. As if satisfied with its two kills, the creature slips placidly back beneath the surface of the water. It is surveyed from above by one of Bellal's people, and from the surface by Jill, Galloway and Hamilton.

Underground, Bellal draws representations of the symbols that are cut into the walls of the City. Sarah recognises them from her earlier visit. The Doctor has 'a rough idea of what they're intended to convey'; he says he has seen them before, on the walls of a temple in Peru. Sarah is incredulous, but the Doctor explains: 'It's one of Earth's great mysteries, that no primitive man could possibly have built such a structure. Well, now we've solved it.' He turns to Bellal: 'You say that your ancestors were travelling in space when other worlds were still primitive.' Bellal confirms this. 'Then,' says the Doctor, 'they almost certainly visited Earth and taught the people there how to build.' Bellal explains: 'Our City supports itself in two ways: through roots in the ground, and from the air.' 'Yes,' says the Doctor, 'they must drain their electrical energy from the atmosphere, probably via that beacon.' 'So that's what put the mockers on the TARDIS,' Sarah realises. The tunnel-dwelling Exxilon who observed the attack at the diggings now arrives with news of the Dalek's destruction. 'Another of our Dalek friends has had root trouble,' the Doctor remarks to Sarah, who smiles coyly back at him.

At the dig-site, a Dalek instructs that mining will recommence immediately in a new area. It orders the Exxilons to move, and Jill to accompany them. Hamilton objects, but Jill reassures him: after all, one of them has to select the parrinium from the digging, and she can take care of herself. The Dalek turns to two others, and orders that the Dalek patrol will leave immediately for the City to conduct its scientific survey; Hamilton and Galloway are instructed to accompany the mission to position the explosive. Since Hamilton is under the erroneous impression that Galloway is legitimately in command of the humans' party, he asks him what they should do. Galloway tells him to comply with the Dalek's instructions.

The City gleams pristine white beneath the bright sunlight, its beacon flashing rhythmically, as Bellal, the Doctor and Sarah arrive near its outer wall. The Doctor whistles admiringly. 'Well, that must be one of the seven hundred wonders of the Universe,' he remarks. 'It's even more impressive when we get close,' Sarah tells him. But the Doctor tells her that she will not be going any closer - he has something far more important for her to do. Sarah feels fobbed off by this, but the Doctor reminds her: 'Sarah, if that Earth rocket doesn't get away from here, millions of people are going to die...I want you to make Galloway and his friends understand they've got to get the parrinium aboard that ship and be ready for blast-off the moment the power is restored. Now remember the Daleks will be fully powered too, and they'll do everything they can to destroy the Earth ship.' 'You think that's what they're intending?' 'Yes, I'm certain of it. I know the Daleks, and they're not medical missionaries.' Sarah agrees to pass on this message, and remarks that Bellal appears impatient; the Doctor and Bellal should go. Sarah is clearly frightened for the Doctor's safety. He tells her to take care, and she says the same to him. He is on the point of leaving when he turns back and adds: 'If I don't come back, you must go with them.' He stops her protest by touching his fingers to her lips, then places his hand on the side of her face, a display of warm affection. She gives a little nod and a nervous smile. As the Doctor and Bellal set off towards the City's outer wall, the beacon pulses remorselessly...

Arriving at the City's outer wall, the Doctor comments on how beautiful it is. 'For you, perhaps,' says a nervous Bellal. 'To us it is evil.' He beckons the Doctor to the wall, and shows him how a section of it lights up with a muted whooping sound when touched. 'Touch-sensitised - that's brilliant!' observes the Doctor admiringly. Around the corner are the symbols whose likeness Bellal tried to draw for the Doctor in the tunnels: a horizontal array of geometric hieroglyphs, composed of circles enclosed in U-shapes at different orientations, which light up when their section of wall is touched. 'I have tried to interpret them many times,' says Bellal. The Doctor recognises that the puzzle is very difficult. Bellal shows him the continuation of the sequence of symbols along the next stretch of wall. The Doctor examines the symbols thoughtfully.

Close behind, the two Daleks who are going to survey the City are approaching.

'I think I have something here,' says the Doctor. 'These symbols don't conform.' Bellal is the first to spot the two arriving Daleks, who open fire. But as the Daleks round the corner and arrive at the section of wall that the Doctor and Bellal have just been examining, they find themselves cheated of their prize - the two companions have vanished. The Daleks look at each other impotently. 'Continue search.'

The Doctor and Bellal have slipped inside the City. Propped upright in the bland white room they have entered are the skeletonised remains of an Exxilon. 'What did you do?' Bellal asks the Doctor. 'I simply picked out the symbol that appeared to be different, and traced its outline on the wall.' 'And that made it slide open?' 'Well, I can't think what else.' 'Can we get out again?' asks Bellal. 'I should think that is extremely unlikely,' replies the Doctor, examining the Exxilon skeleton. 'He didn't get out again, did he?' But the Doctor refuses to accept that this is simply a trap; the other visitors who came to grief in this room - some of whose remains appear to be centuries old - simply failed to pass the next intelligence test. The Doctor does not yet know what this next test will be, though 'logically there must be one.'

Sure enough, there is a large square design marked on one wall, which the Doctor recognises as a maze: 'a test of skill and logic that we have to pass before we can move deeper into the City.' 'And if we fail?' asks Bellal. 'Well then we shall simply add to these decorations,' replies the Doctor jovially, meaning the ancient corpses scattered about the room. The Doctor identifies a point of entry at the top right corner of the maze, and an exit at the bottom left. The maze, like other sections of wall, is touch-sensitised. 'Here goes,' says the Doctor. He touches his finger to the starting-point of the maze and begins to trace out a route through it.

While the Doctor is working on this second puzzle, the two Daleks stand at the City's outer wall, busily scanning the first. 'Computer eye indicates the symbols provide a simple logic test. We must deduce which symbol does not conform to all others.' The computer-scan does not take long, and within seconds the Daleks have identified the odd symbol out.

But inside the first room the Doctor is also doing well. He completes the maze without error, and a large section of the wall containing the maze slides up, allowing him and Bellal to pass through. They cautiously move into the corridor beyond, and the wall slides shut behind them again. They move on.

A panel in the outer wall of the City slides up, allowing the two Daleks to enter the room that the Doctor and Bellal have just left. 'Locate next access point.'

The Doctor believes the floor pattern is another test. He squats down to examine it more closely. 'The people who built this City - they didn't go in for ornamental floors, did they?' He passes his sonic screwdriver over the pattern, and two different types of electronic squeaking sound are heard. The Doctor asks Bellal if he has ever tried Venusian hopscotch, then retracts the question as being silly. He tells Bellal to do exactly as he does, then starts making his way across the floor pattern, using the sonic screwdriver to identify which panels are safe to step on, and treading only on them.

Close behind, the two Daleks have just solved the wall-maze, and glide into the corridor beyond.

The Doctor has crossed the floor design, and is standing at the far side, talking Bellal across. The important thing is not to step on the red panels in the design, just the white ones. He takes Bellal's hand, and on the count of three, helps him jump across the last half of the pattern. Now the Doctor demonstrates why such caution was necessary: he takes a five-piastre piece from his waistcoat pocket and tosses it onto one of the red panels. A blue thread of electricity crackles down from above, and most of the panel explodes in a searing burst of white flame, taking the coin with it. 'Tilt,' the Doctor says, to Bellal's bemusement. 'Right, let's go and see what other pleasures they have in store for us.'

The two Daleks, still just one step behind, now arrive at the corridor with the booby-trapped floor design. 'Proceed with all caution,' warns one. 'This territory will be classified as hostile.' The first Dalek to cross the design is assailed by crackles of blue electrical charge from above; it is alarmed and disorientated, but reaches the other side apparently unharmed. The second Dalek, which still has to cross the pattern, blasts the patterned part of the floor with bullets. Jagged holes are torn into the floor, and the Dalek crosses safely. The first Dalek announces that its non-conductive shielding is burnt out, and that its sensors register it received a seven-thousand volt electrical charge: 'weapon evidently designed to destroy humanoid tissue.' As the Daleks watch, the floor heals up behind them, the holes fading away to leave a perfect patterned surface once again. 'This evidence suggests that City is indestructible. Proceed.'

The Doctor and Bellal arrive in a seemingly empty, featureless room. 'Moving inside the City is like being inside a living thing,' observes the little Exxilon. 'Yes,' the Doctor agrees thoughtfully. 'Its bloodstream, rather than its stomach. We're being moved around like microbes, Bellal.'

Meanwhile, in another room, a motionless figure sits before a monitor screen, apparently watching the Doctor and Bellal and listening to their conversation. 'And all the time getting closer to the heart,' the Doctor is saying.

It is dark again outside the City, and Sarah, nervously dodging a patrolling Dalek, finds Jill and wakes her up. Jill is pleased to see her, but Sarah tells her to keep her voice down. They speak in hushed voices. Sarah tells Jill what the Doctor is doing: 'He's going to try and black out the beacon.' And she passes on his message: 'You must be ready for take-off the moment the power comes back on. Remember, it comes back on for the Daleks too.' Jill, pausing momentarily as the Dalek passes, tells Sarah that they now have more than enough parrinium, collected in bags. The problem now, as Sarah identifies, is to find some way to get it aboard the humans' ship. Jill is pessimistic about their chances with a Dalek standing guard over them, but Sarah is determined to try somehow.

Still apparently observed on the monitor screen, the Doctor and Bellal feel their way around the walls of the empty room. 'Nothing,' says the Doctor frustratedly. 'Absolutely nothing. A room without a clue.' But he knows that since everything in the City has a function, the room must have some purpose. 'Perhaps we have come as far as we are permitted,' Bellal suggests. But the Doctor does not believe this, and says they will have to start examining the room all over again. He asks Bellal to look in one of the corners.

Bellal does this, but suddenly becomes disorientated by an onslaught of flashing light and pulsing sound which only he seems to experience. The Doctor, examining the floor with his sonic screwdriver, looks up just in time to see Bellal pointing a Dalek gun at him. He holds up his sonic screwdriver and activates it, apparently to counteract the mind-control to which Bellal has succumbed. The sound from the screwdriver seems to help Bellal struggle against the force which has possessed him. 'Bellal, don't! Bellal - Bellal, don't, now listen to me,' says the Doctor. 'Don't fire that gun. Now remember why we're here. Remember why we came to this City.' He switches off the sonic screwdriver and advances slowly towards the little alien. 'Bellal, I'm your friend. Your friend, you know that. Don't fire that gun - ' He reaches Bellal and drags the gun from his hands. Freed from his temporary possession, a terrified Bellal tells the Doctor about a blinding light that shone in his eyes. A section of wall slides up, freeing them from the room. 'We seem to have qualified for the next test,' observes the Doctor. He asks if Bellal feels ready to go on; Bellal doesn't, but knows that he must. The Doctor leads him gently from the room. The section of wall slides shut behind them - and the two Daleks enter the room from the other side, having missed the Doctor and Bellal by only a moment.

The City's beacon pulses out white light, rhythmically illuminating the night sky. At the base of the City, a Dalek instructs Galloway and Hamilton to climb to its summit and place explosives on the beacon. The two men are told that Jill, the Daleks' hostage, will be exterminated if they refuse. They are also warned that they will be in range of the Dalek's fire at all times. 'Let's get on with it,' says Galloway. They start climbing the structure.

Sarah and Jill are crouched down, one on each side of a boulder. They talk furtively when the Dalek guard passes out of earshot. Sarah asks Jill if there are any empty parrinium bags. There are some, over by the workings. Sarah has had an idea; she tells Jill to sneak over and get the empty bags, while she keeps watch. Jill does so, but has to pass some Exxilons to get to the bags; the Exxilons seem placid and do not attack her, but their mumbling, grunting noises risk attracting attention. Jill gets back with the bags, and the Dalek glides past a moment later. Sarah tells Jill they need to fill the bags - anything will do. If the Doctor manages to restore power, they have to be ready.

The Doctor and Bellal are deep inside the City. The Doctor suspects they must be getting very close to its centre. Bellal points out that they have come quite a way without any tests, and may be safe now; but the Doctor is not so sure. 'Have you any idea why the City should be testing us in this way?' he asks. (Their progress along the corridor appears on the monitor screen before the silent observer.) 'They could have destroyed us dozens of times. Instead, they gave us a chance to survive by using our intelligence.' And the Doctor's guess about the purpose of all this: 'By passing these tests, we've proved that we have an intelligence level that could be useful. We might have some knowledge or science that they could add to their data banks.'

They turn a corner into a new area. A cowled skeleton is slumped against one wall. The lights become dim. 'If I'm right,' says the Doctor, 'the ultimate test will be an assault on our sanity. So be ready for it.' In confirmation of this, they are assailed by a psychedelic onslaught of sound and light. The Doctor gasps repeatedly to Bellal to fight it. 'It's all an illusion. It's an illusion. You have no substance - no truth,' the Doctor declares to the force invading his mind. 'You do not exist. You do not exist!' The Doctor's own words seem to echo into silence as the force fades away, leaving him exhausted. Bellal too is drained by the attack. The Doctor helps him to his feet. They watch as a new doorway opens for them: this time a section of wall simply fades away into nothingness, creating an exit from the room.

They move into the area beyond - then stop short as they see, from behind, the seated figure which has apparently been monitoring their progress through the City. As they watch, the figure slumps into its seat, disintegrating as it goes. The Doctor and Bellal move forward cautiously. 'Our entrance must have created a current of air, which broke the surface tension,' the Doctor says. The figure has been reduced to a layer of dust in its chair.

As well as the monitor screen, there is other technical machinery in the room. 'This is the brain of the City?' Bellal asks. 'The brain and the nervous system, that we've got to destroy,' the Doctor confirms. Bellal points out that the City defies all attempts to destroy it; he does not believe that they will be able to end its power. But the Doctor thinks he knows a way. He pulls out two circuit boards from a unit and places them in front of him.

Bellal looks nervously around. Two large areas of the wall light up with a pinkish red glow. It takes Bellal a while to turn and see them; when he does, he gives a start and calls to the Doctor, who is irritable to have his work interrupted. As the two of them watch, monstrous shapes begin to appear in the glowing capsules. 'We haven't got much time,' the Doctor says. 'The City is creating antibodies, they're trying to neutralise us.' He tells Bellal to let him know the moment the creatures are completely formed, and hurries back to his work.

It is still pitch dark outside. Galloway and Hamilton arrive at the wind-swept summit of the City, at the base of the beacon's main support. Hamilton says it should be close enough if they set the explosive charges on this support, and he sets the detonator on one of the two boxes and fixes the box into position. Galloway, however, insists that just one charge will be enough to damage the beacon - he wants to save the other one.

Down on the ground, the Dalek that has been guarding Jill instructs a group of Exxilons that work is to commence at dawn. Grunting, they move reluctantly off. The Dalek now realises that Jill has escaped. It reacts with panic, declaring its own failure and activating its self-destruct mechanism, rotating wildly, its voice winding down to a slurring stop.

The Doctor is hard at work on the circuit boards, while Bellal observes the completion of the formation of two hulking humanoid antibodies. 'Doctor - I think you should come now!' The Doctor has nearly finished. The antibodies, looking like imperfectly made clay statues, lumber clumsily from their stands. Bellal asks the Doctor what he is doing. 'Psychological warfare,' he replies. 'I'm trying to destroy the City's brain - engineer what in human terms is called a nervous breakdown.' He is in the process of explaining that a computer, as a machine of logic, cannot stand paradoxes, when the antibodies grab him and Bellal, and drag them roughly away from where the Doctor has been working.

Now, finally, the two Daleks catch up with the Doctor and Bellal, and their arrival is unexpectedly timely. They confront the two antibodies, which advance upon them. The antibodies ignore the Daleks' orders to halt. The Daleks spray them with bullets, but bullets only make them flinch momentarily. An antibody pulls the gun off one of the Daleks, and breaks it. The antibodies batter at the Daleks, forcing them into retreat.

Meanwhile the Doctor and Bellal make their escape. The floor pattern is now safe to run across. Fluctuating lighting indicates the City's disorientation.

The Daleks can make no impression on the antibodies. They beat a speedy retreat towards the City's entrance, the one Dalek that is still armed covering the other, as the antibodies lurch relentlessly after them.

The Doctor and Bellal arrive at a section of wall that is moving erratically up and down, as if uncertain whether to open or close. The Doctor's sabotage of the City is taking effect; its controls are beginning to break down. The section of wall proves more of a problem to the escaping Daleks, as they cannot duck underneath it. But the Doctor and Bellal finally escape out of the City, where it is daylight again. Bellal is relieved, but the Doctor knows the battle is not over yet - the Daleks will still do everything they can to prevent the Earth mission ship from getting away. He urges Bellal to come with him.

The Daleks, with Galloway and Hamilton still captive, await the destruction of the beacon by the explosive device, and the restoration of power. 'Prepare for immediate takeoff. The prisoners will load the parrinium aboard our ship.' Hamilton asks about Jill, and is told she escaped when it was dark. He looks grimly pleased to hear this. The two men start loading bags into the Daleks' saucer.

Bellal and the Doctor arrive, and observe this from behind a large rock, but there is nothing they can do to stop it. 'I know the Daleks,' says the Doctor, 'and they'll cause the maximum havoc and destruction before they go.' 'Why?' asks Bellal. 'It's their scorched planet policy,' the Doctor explains. Sarah and Jill arrive from the Earth ship, with news that it is all set for takeoff the moment power is restored. But Jill cannot pilot the ship. She will need Hamilton and Galloway - who are currently occupied loading parrinium bags into the Daleks' saucer.

In the saucer, standing with Galloway, Hamilton looks at his watch and counts off the seconds until the detonation of the charge attached to the beacon's main support -

- and the support is toppled by a tremendous explosion at the summit of the City -

- and the power in the Dalek ship comes back on immediately.

'They've destroyed the beacon!' Jill cries jubilantly, looking up at the City's summit with the Doctor, Sarah and Bellal. The Daleks arrive, cutting the celebrations short. Most Daleks enter the ship, while one remains behind. 'Well don't prolong the agony,' says the Doctor defiantly, while Sarah holds a terrified Bellal. 'It's obvious you mean to kill us, why don't you get on with it?' 'It is not necessary,' is the Dalek's reply. 'You will perish with the rest of the creatures.' The Dalek explains: 'We have all the parrinium we need. With it we can force the space powers to accede to our demands. If they do not, millions of people on the outer planets will perish.' Hamilton points out that now the power barrier has been broken, Earth can have another ship on Exxilon in less than a month. 'I imagine the Daleks have already taken that into account, Peter,' says the Doctor. 'Correct,' the Dalek confirms. 'When our ship is in space we will fire a plague missile onto the surface of this planet. The plague will destroy all life and make further landings here impossible.' The Dalek backs into the saucer as it delivers this news. The door of the ship starts to close. 'Galloway, for God's sake use the bomb!' yells Hamilton desperately to his comrade still inside the ship. The Doctor ushers everyone away to avoid being caught in the ship's exhaust blast. It lifts off in a cloud of white dust.

'Well,' says Sarah as they watch the Dalek ship depart, 'it's not much consolation, but at least they don't have the parrinium.' She explains to the Doctor: 'The Daleks have only got bags of sand. Jill and I loaded the real parrinium aboard the Earth ship.'

Hamilton regrets ever trusting Galloway. With the Dalek bomb that he insisted on keeping, they could have damaged the Daleks' ship. 'If I knew where he was...' he says furiously.

In the ship, a Dalek orders preparation of the launch of a plague missile. Unseen by the Daleks, Galloway is hiding behind the bags of sand with the remaining explosive charge. He sets the detonator, and presses the button -

- and the Dalek saucer explodes, white fire spraying outwards from the buckling wreck.

'Galloway,' breathes Hamilton. Jill realises Galloway must have hidden away on the Dalek ship when he and Hamilton were putting the bags on board. 'Well, he did his duty,' says the Doctor solemnly. 'Now you must do yours. Get that parrinium to where it's needed.'

'Doctor - look!' calls Sarah. 'Look at the City.'