A disheveled man wearing a strange metal helmet walks jerkily in a ruined landscape. He rips part of the helmet off his head with a scream, then walks calmly into the nearby river up to his waist. He then drowns himself in the water and his body floats quietly away.

Inside the TARDIS, the scanner is still not registering clearly. The Doctor cannot tell where they are, only that there may be moving water outside. Susan checks the instruments and finds the readings all normal. They are in fact Earth readings. Excited, they all exit. They have arrived near the same point at which the man had previously entered the river, high up on the bank and underneath a crumbling bridge.

A quick look round shows them that this is indeed London. Ian and Barbara are very excited at the prospect of finally being home. However, this does not look quite like their London - it is quite messy and seemingly deserted. The Doctor wonders about the time period and what may have happened here, worried about the decay and neglect.

Susan climbs a nearby bridge support to get a better look round, but cannot see anything. Suddenly she falls, injuring her ankle. The others barely manage to get her away as a large section of the weakened bridge falls down where she was. A large steel girder blocks the entrance to the TARDIS.

Ian and the Doctor inspect the ship and find themselves unable to shift the girder. They would need help for that, and the Doctor worries about what others would say about them trying to break into a police box. Ian suggest an acetylene torch instead and spots a warehouse nearby which might hold some tools. The Doctor agrees reluctantly. Both he and Ian both are worried. This is very likely not Ian's time. There have been no sounds, no people, nothing in all the time they have been here.

Barbara has examined Susan and found her ankle only twisted and not broken. The Doctor is angry at her for her impetuousness and the damage it has caused. Because of her ankle, Susan is unable to go with them to the warehouse and they are forced to split up - Barbara staying to tend to Susan. She goes to the river to wet her handkerchief to help ease the swelling of Susan's ankle.

Ian and the Doctor reach the warehouse. They have neither seen nor heard anyone. The building looks long neglected and quite sinister. Doors are open, windows are out. Ian calls out with no response, as expected.

Barbara returns from the river and sees a large sign plastered on a bridge support - it is an emergency regulation forbidding the dumping of bodies in the river. She knows this is not the 1960's now. Frightened, she returns to Susan and tells her what she's seen. Susan, for her part, is glad that they are not home as they will all continue to travel together. But Barbara is saddened. She returns to the river for more water.

The Doctor and Ian continue searching the warehouse, moving to the upper storeys. Unseen, they are watched by another pair of eyes. The place is musty, unused for years. Through a window, Ian sees the top of the Battersea Power Station, 2 chimneys of which have been broken off. More mystery. The Doctor finds part of the answer in a battered old desk - a calendar which gives a date of 2164.

At the river, Barbara bends to wet her handkerchief, but stops short when the body of the man with the helmet comes floating by. She hurries back to Susan, only to find the girl gone. A dirty, disheveled man approaches and startles her. Barbara asks about Susan and the man says she is safe with a man called Tyler. Behind him, there erupts the sound of gunfire. The man spooks and runs, Barbara following.

In the warehouse, the Doctor clears out some boxes, looking for cutting tools. A large box topples over and the dead body of a man falls out. He is wearing another of the metal helmets, which immediately attracts their attention. Ian suggests it is a medical aid, perhaps to help knit bones together, but the Doctor believes it is an "extra ear" to help the person receive high-frequency transmissions. Ian finds a whip in the man's possession while the Doctor finds the cause of death: a knife in the back.

Ian hears noise outside the office and both of them go to investigate. Ian kicks open doors and makes loud noises to frighten off whoever is out there. There is no sign of anyone. Ian kicks open another door, which leads out onto a landing which is long-since gone. Propelled by his own weight, Ian falls out the door and between the remaining support beams. He only barely manages to catch himself on one of the beams. The Doctor hauls him back inside to safety. They both decide to abandon their search and return to Barbara and Susan. From hiding, a young man watches them go.

Out along the river, Barbara follows the two men, the one called Tyler carrying the injured Susan. They run crazily through the ruined buildings, avoiding out of the streets and staying in the ruins and the shadows. There is much ducking and hesitation. The men are clearly frightened of being caught by someone. Barbara follows as best she can.

As Ian and the Doctor emerge from the warehouse, they hear a strange noise in the sky. Looking up, they are alarmed and amazed to see a silvery flying saucer overhead.

At a resting point, Susan and Barbara are reunited. Tyler listens to their story about their two friends. They urge him to search for them, but he refuses, saying they must move on.

Ian and the Doctor return to the river side and find the women gone. They figure they must have hidden somewhere due to the gunfire they heard earlier. They must wait for them to return. Ian is anxious to leave this place, but the Doctor is curious as to what has happened to London. Perhaps a plague or some other disaster. Ian paces in frustration.

The ragtag group reaches a secret tunnel area. Tyler puts Susan down and activates a hidden switch and reveals an entryway to some underground hideout. A young man called David exits from the hideout, angry with Tyler for being late. He sees the two women and offers them safety. As long as Barbara can make herself useful by cooking, they can stay. David tells Tyler of his struggle with a "Roboman" who was waiting in their storehouse. They will need to find another now. Tyler mentions Susan and Barbara's friends who are missing and David recognizes them as the men he saw in the abandoned warehouse.

The conversation is interrupted by a cranky and abrasive man in a wheelchair who joins the group. His name is Dortmun. Tyler tells Dortmun that "they" have landed a saucer at the heliport, and Dortmun seems pleased. He is also pleased to hear that there are more able-bodied men for the cause and he allows David to go and find the Doctor and Ian. He encourages the young man to return soon, as he is going over "the attack plan" soon. However, Dortmun will not answer any of Susan's questions about the attack. Susan and Barbara are taken below while Dortmun stays on watch.

Ian and the Doctor continue their wait for the others. Restless, Ian wanders off and finds the poster about dumping bodies in the river. The Doctor thinks it's a silly sign, but Ian is aghast. He wonders about the Doctor's plague theory.

From above, David sees them just standing in the open. He also sees a group of the helmeted men - Robomen - coming after them. He is too late to do anything.

The Doctor and Ian are flabbergasted as they watch the Dalek approach. The Dalek addresses the Robomen, upset that humans have gotten so near the river. Apparently the leader of this patrol is missing, leading to the confusion.

The Doctor demands release and the Dalek refuses. The Doctor decides to pit his brain against the Daleks and defeat them. He speaks openly of defiance and the ultimate defeat of the Daleks, further angering the creature. It insists that the Daleks are already the masters of Earth and orders the prisoners to be taken to landing area 1.

Susan and Barbara have been introduced to the resistance group led by Dortmun and Tyler. They listen with the others to a Dalek broadcast urging the "survivors of London" to surrender to the Daleks or be destroyed. A girl called Jenny - bitter and war-weary - tends to Susan with little compassion and gives Barbara a job distributing food. Susan, too, will have a job assignment, whether her ankle heals or not.

Dortmun and Tyler discuss the planning of the upcoming attack. Dortmun is sure an attack can succeed, but Tyler is sceptical due to a lack of manpower. Dortmun proudly shows off his newest bomb, which he is certain will destroy the Daleks. David returns and joins them. He tells them what happened to the Doctor and Ian by the river. The extra manpower could have been quite useful. He believes they were taken to the Dalek saucer which landed at their heliport in Chelsea.

At the heliport, the Doctor and Ian await the arrival of other prisoners. Outside the saucer, they muse upon the situation. The Doctor believes that the events of their previous meeting with the Daleks on Skaro happened far in the future. This is perhaps the middle history of the Daleks. It is clear that they are different than the others - free movement on any surface and greater mobility. They are an invasion force adapted to the new environment.

Two more prisoners arrive, one of them responsible for the murder of 2 Robomen. The younger of the two newcomers is adamant that he will not go into the saucer and will not return to "the mine". He bolts and is killed by the Daleks. The Doctor, Ian, and the other man are led into the saucer.

David has informed Susan of the capture of her friends. He wishes not to tell Barbara, as they are to attack the saucer soon. With luck, the two men will be freed and Barbara will be spared unnecessary worry. Susan reluctantly agrees.

Jenny brings in several Robomen helmets. In response to Barbara's questions, she and David explain that Robomen are human beings whose brains have been surgically altered to serve the Daleks. The helmets are for communication. However, "the transfer" as it is called is not permanent. When the processing wears off, the Robomen go insane, smashing their heads against walls or tossing themselves into the river. Barbara realizes that is what she saw earlier and shudders at the memory. Susan has an even worse realization: the transfer operations take place on board the saucers. The same place where Ian and her grandfather have been taken.

Inside the saucer, the Doctor admires the ingenuity of the construction. Ian thinks it escape-proof, but the Doctor is not so sure. They are led into a cell, along with the other prisoner, where the Daleks observe them with a hidden camera. The Doctor's talk of resistance is not unusual to them, but they believe his words betray a greater intelligence than usual. They decide to give him "the test".

In the cell, the Doctor and Ian discuss possibilities for escape, but their cellmate - Jack Craddock - has no hope that they can get out of the cell, let alone the saucer. Ian encourages Craddock to tell them how the Daleks came to invade Earth. He explains that the planet was bombarded with meteorites about 10 years before. At first no one thought anything of it, until a strange plague broke out. The entire populations of Asia, Africa, and South America were wiped out before a cure was found.

In the underground hideout, Susan and Barbara are hearing the same story from David. After the plague, human societies were splintered into small groups unable to communicate with each other and too small on their own to resist the Daleks when they invaded. Many humans were rounded up for use as Robomen. Many others were shipped off to a giant mineworks the Daleks set up.

Craddock finishes, explaining that humans have become demoralized by the ease of their defeat and the fact that the Daleks have used their own kind as soldiers against them. Ian presses for the answer to why the Daleks invaded, but Craddock is unsure. They are digging for something, having turned the whole of Bedfordshire into a giant mineworkings, but he doesn't know what they're after. However, the Doctor may have found what he was after.

The rebels listen to yet another Dalek broadcast, but this one seems different. They say this is the last chance to surrender and live. If not, London will be destroyed and all remaining rebels with it. This news galvanizes the rebels into action. Dortmun calls them all round and explains the plan to attack the saucer.

There are many questions from the group, backed up with large doses of scepticism. But Dortmun is confident, both in their abilities and in the effectiveness of his bomb. But to use the bombs, they must get very close to the Daleks. Barbara comes up with the idea of using the Robomen helmets to trick the Daleks - that way they can escort "prisoners" right into the centre of the landing pad. All agree to this and the attack is set for one hour from now.

In the cell, Ian and the Doctor examine a crystalline box, inside which is a metal bar. Using a similar metal bar which he found earlier, the Doctor deduces that both bars are magnetized. If they can get both of them, they might be able to force the cell door open. Craddock is sceptical, but does not object when the Doctor borrows the magnifying glass Craddock found earlier. Using the glass and a strong overhead light, the Doctor opens a section of the box and the metal bar inside is drawn to the other and out it comes. Both Ian and Craddock are astounded.

The Doctor uses his knowledge of the Daleks' affinity for static electricity and the two magnets to set up a force field around the cell door. The lock snaps to and the door opens. However, the Daleks are waiting for him as he exits and he is captured by Robomen. Ian and Craddock are locked back in the cell while the Doctor is taken away to be "robotized".

Outside the saucer, night has fallen. David, Susan, and Barbara wait in the shadow of nearby buildings for the others to arrive. All 3 are armed with Dortmun's bombs.

A group of rebels disguised as Robomen bring their "prisoners" to the landing area. Unfortunately, the Daleks figure out the ruse very quickly. At that moment, the attack begins, the rebels ringing the area tossing their bombs at the Daleks. They make a lot of smoke but do no damage at all. In the confusion, a number of the fake Robomen and prisoners make it up the ramp and into the saucer. Several do not as the Daleks begin firing.

In the saucer, the rebels spread out, Tyler directing them to release all the prisoners before exploding their bombs inside. Elsewhere, the call goes out for Robomen reinforcements. However, the Robomen in charge of the Doctor's transfer operation are instructed to continue their work. The Doctor is now unconscious and a large section of machinery hovers close over him.

In their bunker, Dortmun and the remaining rebels can hear the sounds of battle, with far more Dalek fire than bomb explosions. It sounds bad.

Tyler and another rebel find the Doctor. They kill the Roboman standing over him and stop the transfer operation. However, he is still unconscious. The rest of the prisoners are freed and head for the exits. The rebels begin using their bombs inside the saucer while Tyler tries to rouse the Doctor.

Jenny joins David, Barbara, and Susan in the shadows of a nearby building. David is out almost immediately to help the others in their retreat. Barbara follows, learning by accident that Ian and the Doctor are in the saucer. Jenny drags Susan away with her.

Rebels and released prisoners flee from the ship, the rebels using their bombs to at least create a smoke screen. Tyler drags the Doctor out, while Ian makes his own way out. The Daleks fire indiscriminately, killing many of the humans. Barbara spots Ian in the smoke and calls to him, but she is forced to leave him when a Dalek bears down on them. Barbara runs but Ian retreats back up the ramp and into the saucer.

Soon the rebels are on the run, Daleks in pursuit, refusing to take prisoners. Many are killed as they run. The Daleks secure the landing area and mop up the rest of the humans. Ian, hiding in the ramp housing underneath the floor plates, listens as they order the destruction of all humans.

Later, in the rebel HQ, Barbara recovers from her wounds. The place is almost empty as the survivors leave. Dortmun sits stony and silent until Tyler arrives. He speaks the unspeakable: Dortmun's bombs were useless and nearly everyone was killed. Barbara asks about the Doctor and Ian. Tyler explains that he and the Doctor got separated in the confusion. He is not sure what happened to him after that.

Tyler binds up a shoulder wound, then announces he is leaving London heading north. The city will be much too hot for them now that the Daleks have been openly attacked. Dortmun wishes to stay to perfect his bombs. Jenny wants to leave as well, but Barbara will stay, hoping to catch up with her friends. Dortmun suggests moving to the London Transport Museum, an alternate HQ. There are facilities there to work on the bombs. Barbara and Jenny agree and they set out.

The Daleks have received orders to destroy London with firebombs. The Robomen will accomplish this while the saucer departs for the mineworks. Ian, trapped onboard the saucer, cautiously emerges from hiding as the ship takes off for Bedford. He is accosted by a Roboman dragging a badly beaten prisoner to a cell. The Roboman is Craddock. He attempts to knock Craddock out, but he seems much stronger after his robotization and Ian cannot get the better of him. The injured prisoner joins the fight and the two of them struggle with Craddock. In the end, Craddock's helmet touches some of the nearby robotizing equipment and he is electrocuted.

Ian introduces himself to Larry and the two men decide what to do. They will go with the ship to Bedford, Larry wanting to see if he can find his brother who was taken there some time ago. They put Craddock's body into a waste disposal chute to hide it. However, the loss of the Roboman is detected by the Daleks and an alarm is sounded. Ian and Larry head for the ramp housing to hide.

Meanwhile, David and Susan have found each other in the aftermath of the attack. They hide out near the entrance to the underground. They are nearly found by a pursuing Dalek, but it heads off without spotting them. However, it finds another survivor nearby. Susan and David must listen helplessly as that man is killed, begging for his life.

Badly frightened by her experiences, Susan wishes she could simply get into the TARDIS and leave. David, however, would not want to leave even if he could. Susan tries to tempt him away, but David is adamant. This is his planet. He belongs here and he must stay, no matter how bad it is. This makes Susan wistful for a time and place to belong. She and David draw closer together.

They are interrupted by sounds approaching. It is a man called Baker, who saw David heading this way and followed. He is bearing the insensible Doctor with him. Susan takes care of her grandfather while David bids farewell to Baker. He will head toward the deserted Cornish coast where he should be safe. However, Baker doesn't get very far before he is gunned down by a Dalek.

To reach the Transport Museum, Barbara, Jenny and Dortmun must cross the city and the river via 1 of the bridges. As expected, the city is crawling with Daleks, making their crossing a dangerous exercise indeed. They are nearly caught several times before reaching the museum safely.

Inside, they find the place empty. Dortmun begins work on his bomb immediately. The trick is to find a way to shatter their casings, which is made of a substance not found here on Earth - he calls it Dalekenium. Jenny returns from a recce of the place, having found no one inside. But there are signs that some rebels have been here on their way to the south coast.

Barbara wishes the Doctor were here to help Dortmun. He, too, wishes he could have his new formula checked by another scientist. Barbara is sure he will be heading for the mine in Bedfordshire. Jenny reminds her peevishly that he may well be dead already, but Barbara refuses to believe that. Dortmun sends Jenny out of the room, and then tries to entrust his new formula to Barbara to show the Doctor. She refuses to take it, saying it will be safer with him and that he can explain it to the Doctor when they meet. Dortmun relents and sends her off after Jenny, saying that they will leave immediately for Bedford.

However, as soon as Barbara is gone, Dortmun gathers one of the bombs and the rest of his things and heads off on his own. He leaves behind his notes. Barbara and Jenny return and find the notes. They realize immediately what he is doing and go outside to stop him.

Dortmun wheels himself out into the open and attracts the attention of 2 Daleks. He stands unsteadily, ready to make the ultimate test of his formula. He tosses his new bomb and the Daleks, but he is shot dead before he can see that the new bomb also had no effect on them. Jenny and Barbara return to the museum and forced to hide as a Dalek invades the museum. Luckily, it leaves without finding them.

Elsewhere in London, the Doctor is recovering from the effects of the robotizer, aided by Susan. The numbness is slowly wearing off. Susan tells him that David has planned their route northward. He hopes to meet up with other rebels along the way to Bedfordshire. However, the Doctor crankily asserts his authority, planning instead to return to the TARDIS. Susan thinks this is a bad idea, as they can't even get into the ship without help. The Doctor is quick to notice that she has come to rely on David's advice.

David returns from scouting, saying that the area is still full of Dalek patrols. When Susan asks what next, he turns to the Doctor for advice. The Doctor suggests they go north to meet up with other rebel groups along the way to Bedford, although he says this decision is really up to David. Susan is happy at this change of heart.

On the Dalek saucer, Ian and Larry talk. Larry believes that if they can find out what the Daleks are mining for, they might be able to beat them. His brother has said they are after the magnetic core of the planet, but it sounds too fantastic to believe.

The saucer lands and the Daleks disembark, followed by their Robomen servants escorting a new batch of miners. Ian and Craddock sneak out of their hiding place and out of the saucer via the disposal chute.

Susan, David, and the Doctor hear the sounds of ticking. Susan spots the bomb. The Doctor starts to stand quickly and collapses unconscious, clearly not over the effects of the robotizer. Susan makes him comfortable and joins David next to the bomb as it ticks away.

David tries to prize open the face of the bomb, planning to dismantle it. But it doesn't work as the casing is sealed. He remembers Dortmun's bombs and the acid inside. He disarms his last bomb and pours the acid from it on top of the firebomb and it eats away at the casing. Using a stick, he knocks away the remains of the casing and pulls the ignition timer free. It slowly clicks down to the ignition point and nothing happens. They are safe. Susan collapses into his arms with relief.

David tries to figure their next move. He reasons that the Doctor is still unfit to travel. So he suggests hiding him safely and exploring the sewers for a way out of London. Susan is reluctant to leave her grandfather, but David believes he will be safe as the Daleks will think the area's been destroyed by fire bombs. He convinces her that it is the only way.

Jenny and Barbara look over an old lorry in the Transport Museum. They are trying to get it ready to travel. Most of the exhibits were kept in working order. This is no exception and requires little more than inflating of tyres to be road worthy. They discuss their chances of success, Barbara realizing quickly how complete is the devastation the Daleks have wrought here.

In Bedfordshire, Ian and Larry infiltrate a mining camp. They watch, horrified, as Daleks and Robomen ride herd over human slaves. They are made to pull ore cars and lug rocks by hand. Most machine labor is being done by dirty, disheveled people using primitive tools and back-breaking labor.

Ian and Larry start to return to cover when they are accosted by another man, called Wells. He believes they are escaped workers, yet he covers for them when they are found by Robomen. However, the Roboman is not so easily put off by his story. He forces Ian and Larry to head for Hut 30, where Roboman selection is to take place. When Wells does not move, the Roboman clubs him viciously.

Ian and Larry return to help him, in direct defiance of the Roboman's orders. They take him into a nearby tool shed where he quickly recovers. Ian hides behind the door as the Roboman follows them. Ian knocks him out handily. Wells is sure the Daleks will trace them here - they always seem to know when a Roboman is attacked. He tells Ian and Larry that he is here to meet a man called Ashton, a black marketeer in the camp. Ian thinks this Ashton might have a way for him to return to London, as he wants to get back to his friends. Wells, however, is certain that London has been destroyed.

As Barbara and Jenny pack up to go, they discuss Dortmun's death. While Barbara can understand the sacrifice he felt he had to make, the cynical Jenny thinks it senseless and un-heroic. Barbara shoots back that without Dortmun's sacrifice, they too would be dead. She cuts off the argument and tries to start up the lorry. After some effort, the engine catches and fires. Jenny opens the doors and climbs in as Barbara roars off out of the museum.

Susan and David descend into the sewers despite the overpowering stench. David thinks they are safe, but Susan is concerned when she finds a spent shell casing. She thinks it could be Robomen, but David informs her that it could be something else - a human who is neither for nor against the Daleks, simply surviving at any costs. David checks his gun - only 4 bullets remaining. As they turn to go, a hard emerges from the gloom, leveling a gun at them.

As Jenny and Barbara drive out of the city, they are afraid that a Dalek they have just passed has seen them and alerted other units. Sure enough, they see ahead of them a phalanx of Daleks blocking the road. But, instead of stopping, Barbara gives the lorry some gas and rams through the line, destroying 2 of the Daleks. They are excited at the small victory, but more worried than ever about retaliation.

A report on the lorry is relayed to saucer Alpha Major, which is flying 45 miles south of the mining area. The Daleks on board change course to intercept.

Barbara hears the noise of the approaching saucer and she and Jenny barely manage to stop the lorry and jump clear before it is destroyed by Dalek fire.

In the sewer, Susan and David are delighted to be reunited with Tyler. He, too, has been trying to escape the city through the sewers. He had mistaken the 2 of them for scavengers, several of which he has run into during his short time down below. Another threat is alligators, which escaped the zoos during the plague and found refuge in the sewers. The 3 of them decide to return for the Doctor. Tyler leads the way. Susan and David hang back a moment, wondering if there will ever be an end to the death and the fear here. David is sure there will be, and it will mean a new beginning - the chance to rebuild the world. This idea fires Susan's imagination and David encourages her, saying that she could help. Before they can pursue the idea any further, Tyler beckons them on.

Barbara and Jenny find themselves on foot somewhere between Edgeware and Stanmoor, on the road to St. Albans. Jenny darkly wonders what happens if Barbara doesn't find her friends at the mines, and Barbara doggedly responds that she'll deal with that if it happens. She offers Jenny the chance to go it alone, but Jenny chooses to stay together.

Night has fallen in Bedfordshire. Aside from Robomen, there is something else on the loose out here: an unseen but vociferous creature patrolling the mine area. Larry and Ian sneak back into the area, concerned about the roaring noises. They reach the tool shed where they were earlier and enter, only to find a man they assume is Ashton pointing a gun at them.

He urges them at gunpoint to leave quickly, but both men hold their ground. Ian asks for passage to London. Ashton asks for payment; gold, jewels, precious metals, anything like that. Ian has nothing and Ashton blithely dismisses the request. Wells arrives bearing jewellery in payment for the food Ashton has brought. Theirs is clearly a business arrangement and there is no love lost between them.

Another roar from outside prompts Larry to inquire about its source. Wells calls it the Slyther. It was created by the Black Dalek, commandant of the camp, as a night guard. It roams the camp looking for food - people.

Susan and David reach a junction in the sewer tunnel. Seeing no sign of Tyler, they climb a nearby ladder in search of him. This leads to another section of the sewer to which they must climb down. Susan jumps onto the ladder, which promptly breaks near the top. She stops herself falling into the water, but cannot climb back up. Worse, an alligator swims in the water below, waiting for her to fall.

Susan calls to David for help, but Tyler gets there first, peering in at her from a manhole above. He fires his gun and kills the alligator, then steadies the ladder so Susan can climb up. David arrives and hauls her to safety. After a moment's rest, Tyler leads them upward. He has found the Doctor and will take them to him.

Ian and Larry share some of Wells' food. Ashton berates him for staying. With the jewellery he has provided, he could easily obtain passage out of Bedford and to some safer place. But Wells knows he has his duty. All four men freeze as the roaring of the Slyther sounds again, nearer this time. Despite this, they are unprepared when its three-fingered claw bursts through the shed wall. The creature appears with a roar, dark black and looking like an upright insect. It kills the nearest man - Ashton. The other three run, Wells one way and Larry and Ian another.

Larry tries to dodge as the Slyther reaches them and almost falls down into the pit. At the last minute, he reaches out and grabs hold of a bucket suspended over the pit. Ian jumps over and into the bucket, then hauls Larry up. The two watch horrified as the Slyther tries to jump in after them. It misses, but grabs onto the edge of the bucket and tries to climb in. Ian and Larry use rocks in the bucket to pound on the creature. Eventually, it loses its grip and falls to its death very far below. Larry is all for running, but Ian is worried that someone might have heard them and wants to stay where they are.

At the mine HQ, the Black Dalek is concerned about the lack of a work party in shaft 9. He is informed that the main work there is done. It awaits only a clearing party, which is being organized. He is informed that the waste bucket there is being lowered now.

Ian and Larry are in that bucket as it descends into the Earth.

The Doctor, Susan, David, and Tyler descend into the London sewer. The Doctor is fully recovered. They have come here to escape two pursuing Robomen who saw them above ground. Tyler is unable to pull the cover over the opening so he is sure that the Robomen will follow them down. The Doctor instructs them to allow both to descend so that they can deal with both at once.

Tyler attacks the first while the second is still on the ladder. David tries to aim his gun at the first as Tyler quickly loses ground in the struggle, but he cannot get a clear shot. His gun is knocked away by the second Roboman and he is forced to fight him hand to hand. Both men are losing ground. Susan, desperate, attacks the one fighting David. But she, too, is easily repelled. The Doctor joins the fray, clubbing Tyler's opponent with his walking stick. Tyler then beats him into unconsciousness. The Roboman fighting David decides to get reinforcements and so begins to climb out of the sewer tunnel. Tyler retrieves his gun and shoots him.

The fight over, Tyler turns his gun on the other Roboman, but the Doctor stops him. Their lives are not threatened by him and they should not take his life. The Doctor urges the group onward toward the mines.

Along the way to Bedfordshire, Barbara and Jenny find a house which looks abandoned. A storm is blowing up and they decide to take refuge inside. They are surprised to see two women inside - on old, the other younger. They are dirty and ragged, but clearly this is their home.

Barbara tries to get them to provide shelter, but Jenny is frightened and wants to leave. The younger woman warns them of a pack of wild dogs in the forest here. The two groups interrogate one another. Barbara and Jenny learn that the Daleks are aware of the house and its occupants, but do not consider them a threat. Besides which, the two women make clothing for the workers at the mine. So the Daleks leave them alone and give them food in exchange for their work.

Barbara realizes they don't really have much food and so offers some of what she carries. This seems to seal the deal. The old woman shows them a place where they can sleep for the night. While they put their things away, the old woman has some whispered words with the other. Soon, the younger woman is off with a bundle of clothing to deliver to the mines. Jenny is suspicious due to the storm and the dogs, but the old woman dismisses her - the young woman has made this journey often enough. She will be fine. The other 3 will prepare the meal while she is gone.

Ian and Larry are still descending shaft 9 in the waste bucket. They have been going down for nearly 20 minutes. The heat and the pressure are increasing rapidly. Abruptly, their descent stops. But they are still quite far from the bottom. Ian jumps out of the bucket and lands safely. Larry stumbles as he jumps and strikes his knee on the bucket. He lands hard and cannot walk when he rises. Ian helps him to cover.

The old woman chats idly during supper, talking of London before the Daleks came. She seems nervous somehow. Indeed, she starts up out of her seat when the young woman returns. The two of them huddle in a far corner of the room. Before Barbara and Jenny can move, a Dalek appears at the door, ordering them to follow or die.

The two women are left alone. Pleased, the young woman shows off the food for which she traded Barbara and Jenny to the Daleks. She eats greedily as the old woman watches the Dalek and its prisoners move off.

Ian checks out Larry's damaged knee. It seems all right, but he is still unable to walk. Ian is perplexed by the set up of the mine. There is no modern equipment here and all they seem to be doing is shifting rock. Larry repeats his brother's assertion that the Daleks are after the magnetic core of Earth. Perhaps they haven't yet found the right shaft and are simply burrowing hole after hole until they find it.

Ian leaves Larry under cover and goes to explore. He doesn't get two steps before a group of slaves is led into the gallery by Robomen. He ducks back under cover with Larry and watches as the workers gather wicker baskets from a pile and move off to another tunnel section. Ian and Larry decide to join the work party to see where they go, but Ian must practically carry Larry. They don't get far before they are stopped by a Roboman.

Larry is stunned to see that the Roboman is his brother Phil. Larry tries to get through to him, but Phil's memory is gone. He doesn't even remember his own wife. Instead, he decides they are runaways from the work party who will be punished severely for disobedience.

Ian tries to drag Larry away, but he resists. There is a terrible resolve in Larry's face as he stands unsteadily on his own and lurches toward his brother. He grabs Phil by the neck and begins throttling him, hoping to end his miserable existence. The Roboman pulls his machine guns and shoots Larry point blank, but he does not stop. As he dies, he chokes the worthless life out of his own brother, all the while begging Ian to run for his life.

Suddenly, an alarm sounds as the Daleks become aware of the death of the Roboman. The miners burst into the gallery, taking their chance at freedom. They bear away the bodies of the dead men whilst Ian retreats further into the tunnels.

The Doctor and his party have made camp near the Daleks' mineworkings. Susan is alone, preparing dinner, when David sneaks up on her and playfully thrusts a freshly-caught fish over her shoulder. They wrestle playfully in the grass, laughing. When they finish, a serious moment overcomes them and they share their first kiss. However, they are interrupted by Tyler and the Doctor. The latter hears their babbled "explanations" but only observes wryly that he thought he noticed "something cooking".

They sit down to a meal of rabbit and fish and discuss what they have observed. The Doctor is convinced that the mine is the centre of the Daleks' operations here on Earth. He asks Tyler why no resistance has been organized here. Tyler said it seemed insignificant to them, as they believed the Daleks were only mining for some natural resource. They thought the invasion was merely to conquer humanity. But the Doctor says that humans are only an expendable workforce to them. They are here because there is something unique under the Earth that they must have. They are burrowing so deep as to tamper with the primeval forces of the planet's creation. And the Doctor is determined to stop them.

Ian is forced to hide as another work party passes. He is thrilled to see Barbara among the workers. He follows them. A Dalek watches over the party as they fetch heavy baskets full of rocks. Jenny, tired and demoralized, stumbles and dumps her basket. Barbara is quick to help her. Jenny dissolves into tears, sure that they are beaten and will die here. Again, Barbara must remind her of how far they've gotten. She suggests finding the Daleks' main control room, although Jenny is sceptical that anything can be done even if they do find it.

Also in the work party is Wells. On orders from the Dalek in charge, he gathers empty baskets and returns to the cleaning area. Ian intercepts him and tells him to contact Barbara to tell her he is here. Wells goes off to dump the buckets first.

Barbara hits on an idea. She approaches the Dalek and tells it that rebels are planning a revolution against the Daleks. The Dalek dismisses her until she produces Dortmun's notes. The Dalek recognizes the formula as that of the London bombs. Barbara presses on, saying she has names and locations of the rebels. She refuses to give the information to this single Dalek, wanting instead to see a Dalek leader. It contacts HQ and the Black Dalek agrees to see her. She and Jenny follow the Dalek out.

Ian watches helplessly as she goes. Wells returns, but he is too late. He directs Ian to an empty gallery in which he might be able to lose himself.

In the Dalek control room, the Black Dalek is briefed on progress. The main shaft has reached within 4 miles of the Earth's outer core. All that remains is the placement and detonation of the penetrating explosive. Once the explosion is triggered, the Earth's magnetic and gravitational energies will be eliminated.

Ian finds himself in a Dalek laboratory. Suddenly, 2 Daleks arrive and listen to a message broadcast by the Black Dalek. He hides himself in a small cubby hole near the centre of the room and listens in horror to the message. The Daleks' mission is to remove the Earth's core and replace it with a power system which will enable them to pilot the planet anywhere in the universe.

Ian frantically pulls out wires in the mechanics around him, at last succeeding in stalling the capsule in its descent down the bomb shaft. However, it is quickly hauled up by a rope pulled by a team of Robomen. Ian continues pulling wires and manages to open a hatch in the bottom. He uses this to escape, grabbing onto a ladder which runs along this part of the shaft.

Inspecting the capsule for damage, the Daleks discover the presence of a human and which way he has gone. An alarm is sounded and Daleks begin searching for Ian. One finds him, just as he starts to lower himself further down the shaft on a rope. The Dalek fires and breaks the rope. Ian falls/slides down the bomb shaft, coming to a hard stop at a turning. He hits his head and is knocked unconscious.

Barbara and Jenny are brought to Dalek central control. Barbara instructs Jenny to find the main control panel and try to do as much damage as possible. She will keep the Daleks occupied. As they enter, they hear the end of a discussion regarding the ongoing repair of the penetration explosive. It is the last part of their plan. All other work in the mines has been completed and the Black Dalek orders all humans to be herded to the lower galleries for extermination in the upcoming blast.

Barbara and Jenny watch as a Dalek goes to a control panel and issues orders to the Robomen by speaking into a microphone pickup. Barbara realizes they could give the Robomen new orders - turn on the Daleks - through that panel.

They are interrupted when the Black Dalek approaches. He instructs Barbara to talk, quickly. She starts to spin a yarn of revolution that includes the Boston Tea Party, General Grant, and Hannibal. Jenny tries to get to the panel but cannot. Barbara finally succeeds in inciting a panic in the Daleks, but they see through it when she tries to use the microphone. She and Jenny are captured, to be held here and killed during the explosion. The Daleks return to work.

The Doctor, Tyler, David, and Susan examine the mine crater from a cliff top nearby. The Doctor directs Susan and David to use Dortmun's remaining bombs to destroy an aerial on the other side of the crater. He and Tyler set out down into the mineworkings on a mission of their own.

Ian comes to find himself facing a service hatch into a lower gallery. He opens the hatch and is nearly caught by Robomen hauling timbers nearby. He closes the hatch and waits.

The Black Dalek announces the repair of the capsule and its imminent release down the bomb shaft. Barbara and Jenny are happy to hear that the Daleks will be leaving shortly. They will board the saucers to hover over the crater and await and observe the destruction of the Earth's core.

The Doctor and Tyler reach the Dalek control area. The Doctor neutralizes the warning system on a door and opens it. They both enter.

Hearing the Robomen leave, Ian opens the hatch again. He hurriedly grabs some of the timbers and shoves them out into the bomb shaft, bracing them with the hatch door. Then he hurries off.

The Daleks launch the explosive capsule again, then beat a hasty retreat. Unbeknownst to them, Ian's work has blocked the bomb shaft and the explosive is stopped far about the Earth's core. Soon it will explode.

The Doctor and Tyler reach central control just as the Daleks are leaving. They hide until the creatures pass and then enter. The Doctor and Barbara have a short but joyous reunion. She and Jenny bring them up to speed on events and the nature of the Daleks' plans. The Doctor is very keen to prevent the Daleks from leaving and sets to work immediately.

He goes to a scanner screen and adjusts it to observe Susan and David's progress. They are still working to destroy the aerial, which the Doctor says is key to the Daleks' power supply here on Earth. However, the Daleks are still monitoring the scanner frequencies and detect the change. A Dalek is despatched to central control to deal with the escaped prisoners.

A Dalek is almost immediately en route to the control room with orders to exterminate. It bears down directly on the Doctor, who stands calmly by...just as Susan and David succeed in their task. With its power supply halted, the Dalek overheats and is destroyed.

Barbara tells the Doctor about the Roboman control panel. They use it to order the Robomen to turn on and kill the Daleks. Suddenly, pandemonium breaks out as former slaves and Robomen work together to destroy the remaining Daleks and escape the mines. A flood of human beings pours out of every gallery and shaft, all heading for the surface. Ian heads for the control room, and is reunited with the Doctor and Barbara.

Ian tells them about his sabotage. The Doctor knows that the bomb won't harm the Earth's core, but the explosion will be tremendous nevertheless. Everyone heads for the cliff tops overlooking the crater.

The bomb goes off, and the entire enormous mineworking is filled back in again. The Doctor is overjoyed to see that the Dalek saucers hovering over the crater were caught in the updraft of the explosion and also destroyed. He is certain that no Daleks survived.

Tyler and Jenny watch the crater filling in with stunned amazement. The fight which they never thought would end - the fight they were sure would take their lives - is over. And they have survived. The question of what next is foremost on their minds.

Some time later, the last of the debris is cleared from in front of the TARDIS. Tyler is amazed that much of London survived the Dalek firebombs. He only wishes Dortmun was here to see it. The Doctor agrees, adding that their next task - rebuilding the Earth - must be dedicated to the memory of those who didn't survive the fight. The Doctor is a little wistful at the opportunity before humanity - the task of rebuilding an entire world. The chimes of Big Ben sound again in the background for the first time in years.

Nearby, Susan sits alone, disconsolate. The Doctor joins her and there is an unusual awkwardness between them. They fill the space with idle talk of mending Susan's worn out shoe and the disastrous state of her room on the TARDIS. The Doctor observes that she needs "taking in hand" and excuses himself to go and "check on the ship".

David has observed the entire conversation. He watches as the Doctor hesitates at the door of the ship. Susan goes slowly to David. The Doctor watches for a moment, pain on his face, and then enters the ship. Ian engages David in conversation about his plans, what he will do during the reconstruction. David responds with talk of rebuilding agriculture, but his mind is obviously elsewhere. Barbara realizes what is happening and pulls Ian away and into the TARDIS. Tyler, too, says a quick goodbye and moves off.

Susan and David are alone. He wastes no time in begging her to stay, professing his love for her and proposing marriage. Susan tries to protest, saying that this is not her time and place and that she must look after her grandfather. But David persists, offering her a place and time to belong and a part in the rebuilding of Earth. Even though she professes her love for him, she still cannot stay.

The Doctor, Ian and Barbara watch this on the TARDIS scanner in silence. The Doctor suddenly comes to life, closing and locking the TARDIS doors. Susan reacts in surprise and calls out to him.

The Doctor speaks to her over the loudspeaker system, watching her on the scanner. He has locked her out. He acknowledges the fact that they have long taken care of each other, but she is now a grown woman and must find her future away from him. It is with David. "One day I shall come back," he vows. "Until then there must be no tears, no regrets, no anxieties." He tells her that she must "go forward in all your beliefs. And prove to me that I am not mistaken in mine."

He bids her a heartbreaking farewell and dematerializes the ship.