On the planet Earth, a rocket is launched into space on an orbital atmosphere survey mission. It is a typical enough event, but this orbiter, Zeus 4, will encounter a unique and deadly fate.

Zeus 4's flight is being tracked by the scientists and military personnel of International Space Command's Snowcap Base, a tracking station buried under the ice at the South Pole. In command is an American general called Cutler. He is assisted by Dyson and Dr. Barclay, two civilian scientists. The scientists banter with the two astronauts - Schultz and Williams - as they pass overhead. The flight is routine so far. In the rocket, Schultz and Williams go through routine checks of velocity and altitude. They activate their atmospheric probes and begin transmitting data back to Snowcap.

Outside the base, there is a furious blizzard blowing. It is the coldest place in the world. A periscope and several aerials extend above the ground from the base below, used for surveying weather conditions. The periscope reaches to the guard room of the base, where several soldiers of various nationalities rest between duty shifts. One, an American sergeant, scans the outside through the periscope. It turns in its rotation, just passing by as the TARDIS materializes.

Inside the ship, the Doctor is impressed by the violence of the blizzard. He still wants to go out and investigate and he hurries Polly along with his cloak. She returns dressed warmly from head to toe as Ben suits himself up. Both are impressed by the extent of the Doctor's wardrobe. The Doctor has only a cloak and hat on, but assures his companions that he will be warm as toast outside. He opens the door and out they go.

Ben and Polly are first out, struck immediately by the howling wind and numbing cold. Polly spots the aerials and periscope - the only interesting sights on the landscape - and goes toward the periscope as the Doctor emerges from the ship. Obviously there is something down below the ice. Just as the Doctor and Ben join Polly, the periscope rotates to face them all.

Down below, the Sergeant can't believe his eyes. He calls to one of his crewmates, an Italian private named Tito, to try and confirm what he's seen. Tito sees the trio of curious faces too, one of which is a very attractive woman. He is thrilled. The Sergeant takes over the scope and turns to spot the TARDIS as well - some kind of "blue hut" that didn't used to be there. He orders two guards to arm themselves and head up to bring the newcomers inside. The guards emerge from the base via a trapdoor in the ice. They quickly surround the Doctor and his companions. Ben, the Royal Navy man, knows these soldiers mean business and encourages his friends to comply. All three head down into the trap door.

They emerge in the guard room where the Sergeant is quick to push, prod, and bully them. He hits them immediately with questions. Polly answers first, saying that they just landed outside in a sort of spaceship. The Sergeant is not amused with this response. The Doctor tries to find out where they are. When the Sergeant tells them, Ben and Polly are happy to hear they are still on Earth, and the Sergeant takes this as proof that they are all crazy. He has Tito place a call to the General.

Ben is concerned that the CO here is a general. Obviously, this is a high-security post. The entire guard room snaps to attention when an irate General Cutler enters a moment later, having already been on his way. The Sergeant has difficulty passing on the little information he has, as it sounds so ludicrous. Less than impressed with this report, the General surveys the newcomers for himself. He recognizes Ben's military demeanour and this is confirmed by the lad's snap to attention. However, he cannot adequately explain why he is here and not with his ship. The Doctor tries to assure the General that they are quite harmless, but Cutler says he'll judge that for himself. He and the Doctor take an instant dislike to one another. Too busy to bother with them now, Cutler orders them taken to the Tracking Room and put under guard.

Cutler returns to the Tracking Room himself. The Doctor, Ben, and Polly enter a moment later, escorted by Tito and the Sergeant. The men in the room react with appreciative whistles to the sight of Polly in her mini-skirt, and Cutler silences them with one quick order. Ben and Polly are interested in the technology and the activity going on. It is similar to what they have seen of space shots on TV, yet somehow different. There seems to be a lot fewer people than is necessary. The Sergeant explains that General Cutler prefers to work with a small staff and to work them hard. Soon, most of them will be rotated off this base and a new crew will come in. Ever on the lookout for a way home, Polly hopes for a lift back to England. Ben is sure the General won't let them go easily, and the Doctor points out a nearby wall calendar. It is December 1986, 20 years since Ben and Polly left England. They are still at sea!

Ben and Polly's disappointment is momentary. They are quickly curious again about the activity in the base. They learn from the Sergeant that humans have already landed on the moon and that such expeditions are now almost routine. This particular flight is a simple atmosphere testing probe. However, as they listen to the technicians' chatter, they can all tell that something has gone wrong.

Williams reports that Zeus 4's altitude is 1100 miles as they orbit above New Zealand, instead of 980 as it should be. Dyson rechecks this calculation and confirms the error. Barclay then radios up to the capsule, ordering them to take a visual check on Mars to establish their position while Snowcap does the same.

Schultz complies, finding that the Mars reading is also way out of ligne. Checking again, he finds an even more disturbing result: it is not Mars he's been fixed on, but another planet altogether. Williams laughs it off until he realizes his colleague is serious. He takes the scope and looks for himself. There is a brand new planet in orbit between Mars and Venus. And, somehow, it looks familiar! As the capsule comes in view of dawn over San Francisco, they call their news down to Snowcap. The base responds that they are having trouble reading the transmission, even though their power output is up. But Barclay does read the message and looks to get the new celestial object on his scope. Meanwhile, Williams notices the capsule's fuel cells showing a mysterious power loss.

The Doctor takes in this exchange and is certain of what the scientists will see on their scope. He dashes off a note and bullies the Sergeant into taking him to General Cutler. But Cutler does not believe him and will not listen. Dr. Barclay, however, is intrigued and takes the note just before the Doctor is hustled to the back of the room. Dyson activates the scope and brings the new planet into hazy view. All of the technicians react with consternation. Planets don't just appear, and this one is actually approaching Earth. Barclay and Dyson quickly realize it is the new planet's influence which has drawn Zeus 4 out of its orbit. They decide they must get the spacecraft down right away. Cutler agrees and they go to work.

Barclay contacts the capsule and learns of the 20% power loss in their fuel cells. He tells the astronauts that they need to return to Earth as soon as possible and asks them to verify their coordinates. Once that is done, they prepare for an altitude thrust. However, this action goes very wrong and the ship begins to tumble. Schultz attempts to use the manual controls to stabilize them but doesn't seem to have enough strength to complete the task. Only when Williams assists can they move the controls and stabilize the ship. It is as if something is drawing all the energy from their bodies.

At Snowcap base, preparations for the emergency splash down are in full swing. Dyson orders Rome computer base to compute the final descent path. Barclay reports that Zeus 4's flight path has corrected. Schultz reports their physical energy loss with mounting alarm. They want to be brought down on this orbit, but Barclay has the final descent path now and it is too sharp to begin this orbit. They must hold on for one more. But, as the two men go through pre-check for re-entry, they realize that their flight path has gone off course again.

The new planet's gravity is pulling Zeus 4 out of its orbit. Barclay sets Dyson to calculating a corrected flight path and orders a technician to sharpen the image of the new planet. Curious, he approaches the Doctor and asks what he knows of this. Cutler joins them as the Doctor studies the new clearer image. Satisfied, the Doctor asks the others what they see. One by one, Ben, Polly, and Barclay recognize the land masses on the new planet. They are upside down, but are almost identical to those of Earth! Cutler is sceptical, figuring they must be seeing a reflection of Earth, but Barclay knows there is nothing to reflect off of. And when Barclay looks at the note the Doctor gave him earlier, he is astonished to find written what they have just seen. He tries to get more information, but when the Doctor starts to explain about Earth's ancient twin planet, Cutler ends the conversation by storming off and dragging Barclay with him.

Alone again, Polly notes the Doctor's worried look. He knows what this planet is and what it means to Earth: very soon, they will be having visitors! Ben tries to make light of this, but the Doctor's mood is dark indeed. He silences Ben and then turns his attention to General Cutler as he contacts Mr. Wigner, General Secretary of International Space Command.

Cutler informs Wigner of the new planet and of the situation with Zeus 4. Wigner orders his aides to get him an image of the planet immediately. Cutler also informs him of the three "intruders" at the base and that one of them seems to know a good deal about the new planet. Wigner encourages him to get more information from the Doctor and relay it to him at once.

The General barges over to the Doctor and his friends and begins bullying them, determined to get some answers. He is certain that their appearance at the same time as this new planet is not coincidental. The Doctor offers no explanations to the hotheaded General and Polly protests their innocence. The General loses patience quickly and orders his Sergeant to go outside and search the Doctor's "hut". That, he hopes, will provide him with the answers he needs.

While Tito and the Sergeant get suited up to go outside, a strange disc-shaped spaceship lands in the icy landscape of the South Pole. The Doctor was right - visitors have arrived.

Tito and the Sergeant emerge from the trap door into a blinding snowstorm. They stagger over to the TARDIS, now piled with drifting snow, and attempt to break in. They cannot. The Sergeant sends Tito back inside for a welding torch and some help.

Left alone outside, the Sergeant becomes aware of indistinct figures in the distance coming toward him. He is sure it is not Tito, but does not know who it could be. The three figures get closer and he can now see that they are robot-like, marching in step with no apparent reaction to the numbing cold. Metal attachments sprout from all over their humanoid bodies - arms, legs, heads, chests. They are enormously tall. When they do not respond to his calls, his panic mounts and he pulls out his gun and fires twice. The bullets have no effect. The lead creature reaches him and knocks him down with one swift blow.

A few moments later, Tito and another soldier emerge from the trap door. They see a hooded figure standing by the TARDIS. Assuming this is their Sergeant, they go to him. Unfortunately, one of the robotic creatures has appropriated the dead Sergeant's hooded cloak. It turns and kills Tito effortlessly. The two other creatures come from behind the TARDIS and kill the remaining soldier.

General Cutler has finally taken the time to listen to the Doctor's prediction of visitors from the new planet. He dismisses it as nonsense and returns to the mission of bringing Zeus 4 down safely. He learns that the capsule is still 230 miles off course and is very worried. However, he does his best to hide his fears and present an optimistic front to Schultz and Williams when he contacts them. He then begins bullying his team to make the safe splash down happen.

Outside, the three robotic creatures disguise themselves as best they can with the cloaks of the dead soldiers, then they move toward the open trap door.

In Geneva, ISC General Secretary Wigner is trying to get in touch with Snowcap base, but there is serious interference. He turns instead to a nearby television monitor to catch an international news report, just in time to see a global broadcast of information and a picture of the new planet. The presenter also announces that the planet is moving toward Earth, but will not come close enough to collide. Wigner listens with a mixture of hope and despair. The whole world now knows about this situation and he hopes to himself that there really is no cause for alarm. He tries again to contact Snowcap for more information, but is told by a technician that the interference is enormously powerful and seems to be coming from the base itself.

At the base, tension has risen. Barclay announces that Zeus 4's final orbit will begin in just 4 minutes. He addresses the entire base and asks for a concerted effort from the whole. If the capsule's power falls too low, they themselves will need to take over re-entry. The Doctor hears this and reacts vigorously, insisting that re-entry must occur now as they will not last another orbit.

Going to talk to the General, the Doctor's worst fears are confirmed. He sees the three alien creatures entering the Tracking Room. He tries unsuccessfully to get the attention of first Cutler, then Barclay, his panic rising. Both men put him off. Annoyed, Cutler turns to his guards to drag the Doctor away, but finds instead a gigantic robotic humanoid staring with an icy blankness. He stops speaking midsentence, stunned. The other two creatures throw off their cloaks and stand menacingly in the Tracking Room.

Polly screams in terror and the others begin to panic. Cutler tries to restore order, but one soldier rushes forward with his gun raised. The lead creature pulls a large section from its chest unit which is a ray weapon. The light on it glows blindingly and the guard falls, smoke billowing from his dead body. Horrified, Polly tries to go to him, but Ben and the Doctor hold her back.

In the silence which follows, General Cutler turns to the leader still intent on his mission to bring his astronauts down safely. The creature speaks, harsh and matter-of-factly, in a grating mechanical sing-song. It tells Cutler the astronauts will not return and that the reason why is unimportant. Cutler reacts angrily, trying to grab the creature's arm. It knocks him away easily.

The creature is uninterested in the crew's worry for the astronauts. He does not care in the least. In fact, he is confused. There are people dying all over the planet and people are concerned only for these two. Polly protests that these two could actually be saved if the crew is allowed to do their jobs, but the creature ignores this and changes the subject.

The creature explains that they come from the new planet - Mondas - once the ancient twin of Earth. Aeons ago, it drifted away to the edge of space. Now, they have returned. He tells them his people are known as Cybermen, once just as human as the people of Earth. But their scientists devised metal and plastic replacements for human body parts in an effort to reverse their shrinking life spans. Polly believes this means they are robots, but the Cyberman tells her their brains are still human, but will certain weaknesses - emotions - removed. They cannot feel pain and so have little concern for creatures who do. Engrossed in this discussion, the Cyberman does not notice General Cutler edging toward the communications equipment. He hits a button which sends an emergency signal to ISC Geneva. The Cyberman's bland response holds a barely-hidden air of menace.

At Geneva command, Wigner is trying to piece together events. The energy loss is affecting not only Zeus 4, but the entire planet Earth. As the new planet gets closer, the energy loss increases, meaning that the new planet must be responsible. A technician interrupts him with news of the emergency signal from Snowcap, but the signal has cut out abruptly with no further message. Wigner orders use of the emergency microlink.

At Snowcap base, the Cyberman orders Cutler to relay an all clear to ISC, but Cutler refuses and tells the Cybermen to "take a jump". On orders from the leader, a second Cyberman comes forward and places his hand on Cutler's head. The General collapses immediately and is placed on a bench along one wall.

There is a murmur of disapproval from the crew and Polly accuses the Cybermen of murdering Cutler. The leader tells her he is not dead and will recover shortly. However, it still insists that someone call Geneva. Dr. Barclay adamantly refuses, but Dyson wavers. The Cyberman threatens to destroy the entire communications console and everyone realizes this will mean certain doom for Zeus 4 if they lose communication. The Cyberman keeps up the pressure until Dyson cracks and begs Barclay to do as he says.

Barclay activates the microlink and clumsily reports a mechanical fault which led to the emergency signal. He blames it on the reactor and this seems to satisfy Wigner. He signs off and Barclay nearly collapses onto the console with relief.

Dyson praises Barclay for having done the right thing. Barclay pulls himself together and asks the Cyberman to allow them to contact the capsule. The Cyberman tells them matter-of-factly that they will fail to bring the astronauts down and that they are wasting time. However, he relents and gives them access to the communications equipment. He instructs his comrades to watch for any signs of deceit and to kill if necessary.

While Barclay busies himself on the radio, Ben takes in the scene, looking for some action he can take. He spots the dead guard's machine gun lying unnoticed on the floor. He brings it to the Doctor's attention, hoping to use it to force their way out and back to the TARDIS. The Doctor knows this is foolhardy and doomed to failure, but Ben will not listen to his warnings. He dashes for the gun, but only manages to pick it up when the lead Cyberman spots him. He trains his own weapon on Ben and orders him to come forward. Trapped out in the open, the lad has no choice but to obey. The leader grabs the gun and bends it both stock and barrel, rendering it useless.

One Cyberman is ordered to take Ben away and lock him out. When they are gone, the leader turns to the stunned humans and tells them that Cybermen are "stronger and more efficient" than Earth people. They must be obeyed.

Ben is taken to a nearby room and shoved inside. The Cyberman closes the door and remains outside. Ben tries the door, but it is locked. He looks around and finds that he is in a film projection room with no means of escape readily apparent.

Barclay contacts Zeus 4 and works with them to achieve a safe splashdown. A Cyberman watches over him at all times. Barclay orders a forward correction of 7 degrees. Schultz activates the attitude controls and the capsule reorients itself. Barclay then gives them a 20 second countdown and then they fire the retro rockets to begin their descent. However, it is apparent that their speed has not decreased significantly. Sure enough, a check shows them at 14.5 instead of the required 11.2. Beginning to panic, Schultz fire the retros again, but the burst is short and sputters out. Their fuel is exhausted. Unsure of what will happen, the astronauts begin donning their space helmets.

Watching helplessly on the radar screen, Dyson and Barclay can only relay what is happening - instead of reentering, Zeus 4 is being dragged away from Earth, accelerating as it goes. The Doctor foresees the terrible tragedy as the craft moves away ever faster. Everyone watches on the monitor as Schultz and Williams struggle to do something, anything, to save their lives. Smoke fills their cabin.

Suddenly, the monitor flashes bright white and then goes dead. The base crew hang their heads in sadness. Polly, unsure of what has happened, turns to the Doctor who tells her sadly that the ship exploded.

Ruthlessly, the lead Cyberman points to this as evidence that Earth is in grave danger from Mondas. He says the base crew will be saved from a similar fate, but it requires personal data from everyone. Chaotic conversation fills the Tracking Room as the leader alternately asks and answers questions. He tries to elicit name, age, and occupation information while being pelted with questions. The leader reveals to the Doctor that the energy of Mondas is nearly exhausted and it has returned to Earth to recharge - that is the energy drain experienced on Zeus 4 and elsewhere. Dyson realizes that this means the Earth will die. The Cyberman calmly acknowledges this, eliciting noisy protests from the humans. The Doctor asks how the energy drain is going to be halted when Mondas is resupplied, and the Cyberman admits that it cannot be stopped. However, everyone here will be spared and taken back to Mondas. Indignant shouts erupt again.

In the projection room, Ben prowls restlessly, still looking for a way out. The only "weapon" he's found is a long screwdriver. While the idea of "taking apart" a Cyberman is amusing to him, it isn't very practical. He turns his attention instead to the film projector nearby. He realizes that he could use it to blind his guard and escape. He aims the projector at the door and switches off the lights. He then turns on the projector and calls to the Cyberman. The creature opens the door and is momentarily blinded. Ben manages to leap forward and grab the Cyberman's ray weapon, but cannot get past the creature before it steps out of the light and can see again. Ben is trapped inside, the creature between him and the door.

The Cyberman advances on Ben, who tries to get it to back down with the threat of the weapon. But the creature continues its relentless advance, smashing boxes and equipment as it comes. Ben grows more desperate as he's backed into a corner. With the Cyberman just feet away, he fires the weapon and the Cyberman collapses into a smoking heap. Ben deeply regrets having to take a life, but knows that the creature gave him no alternative. He hurries back to his friends.

In the Tracking Room, Barclay, Dyson, and the others try to argue with the Cybermen, but the leader is adamant that they must come to Mondas in order to survive the imminent death of Earth. When Polly protests that humans are not like Cybermen, the entire group learns that they are to become Cybermen once on Mondas. The leader assures them they will be better than they are now free from disease, heat, and cold. But the humans only know that their emotions and feelings will be extinguished. They will be as good as dead. The leader seems content to continue debating this point, especially with the overly-emotional Polly.

While this is going one, General Cutler comes to and sits up, unnoticed. Looking up, he sees Ben sneaking into the room, still carrying the cyberweapon. He signals up and Ben passes the gun to him. Suddenly, Cutler is on his feet in a flash, firing at and killing the two remaining Cybermen in a matter of seconds. They collapse in smoking heaps.

The General is all business, calling for a connection to Geneva and for someone to remove the Cyberman corpses. The Doctor chides him for acting so rashly as they have now lost their only source of information on Mondas and its inhabitnats, but the General is not interested in the creatures one bit.

Cutler picks up the microlink to talk to Wigner, but he stops short when Barclay tells him of the fate of Zeus 4. Wigner offers condolences on the loss of the astronauts. Cutler tells him briefly about the "visitors" from Mondas. They were dealt with, but the General is shrewd enough to know that more will follow. Wigner agrees to put military bases around the world on immediate alert, but Snowcap will have to hold out on its own for now.

Wigner then relays the news that during the communications blackout, a second capsule was launched with a single astronaut on board to help Zeus 4. Now, Snowcap must try to bring him down safely. Cutler is fine with this until he learns to his horror that it was his own son who volunteered for the mission. Cutler is stunned, accusing Wigner of sending his son to his death. But Cutler's resolve kicks in and he is determined to succeed this time. He is cheered to learn that his son's capsule has double the power reserves of Zeus 4. He might just make it.

While Cutler is occupied with this, Wigner wonders if Earth is about to fight its first interplanetary war.

Cutler goes into action immediately, ordering his scientists and technicians to get in contact with his son's capsule right away. He also orders double of the guard all over the base, stretching security personnel to the limit. He also orders Cobra missiles put on alert for imminent launch. He is taking no chances. The Doctor's disdain for the military kicks in and even Ben agrees that Cutler is overdoing it. The Doctor tells the General that all these precautions will not likely stop the Cybermen next time, but he is ignored. Cutler praises Ben for killing the Cybermen - a good, strong action. Ben responds with regret, which further infuriates Cutler. When Polly notes that the General seems to be enjoying the tenseness and killing, his true rage bursts forth. He tells her about his son being in the rescue capsule and how his death is almost a certainty. He will do whatever it takes to get him back. Polly apologizes.

General Cutler knows what the radar contacts mean more Cybermen. He wants to make the most of what little time he has and orders Dyson to contact Zeus 5 immediately. In the midst of the chaos, the Doctor suddenly moans and collapses. Ben, Polly, and Barclay lower him to the floor, but they cannot get him to respond. Ben tells this to the General, hoping to get some medical help, but he is too busy to be bothered now. He despatches one staff member to help Ben and Polly take the Doctor to a cabin to rest.

Cutler then turns to the communications console as the voice of his son, Lt. Terry Cutler, comes through. He grabs the microphone and asks if Terry is experiencing any power loss. The young man tries to joke with his father, unaware of the tension at the base, but Cutler only repeats the question. There is some power loss when Terry is nearest to Mondas, but it picks up again when he is on the opposite side of Earth. The General tells Terry only that there's been some "trouble" with Zeus 4 and that his mission has been scrapped. All they must do now is get Zeus 5 down.

Ben and Polly have made the Doctor comfortable on a bunk in cabin 7. They cannot figure out why he passed out, except that Polly thinks he seems to be "worn out". Ben thinks he will be OK and suggests they return to the Tracking Room as there is nothing more they can do for him until a doctor arrives. Polly reluctantly agrees.

Back in the Tracking Room, Ben and Polly arrive to hear General Cutler ask his son if he's seen any sign of the formation of spaceships, but he has seen nothing. Cutler orders him to keep a sharp eye out and then assures him they will get him down as soon as possible. It is clear that the blustery General has a soft spot - his son. Desperation and resolve mix clearly on his face. He addresses his staff, spelling out their three major problems as he sees it: getting Zeus 5 down, dealing with more Cybermen, and stopping the drain on Earth's energy. He proposes one solution to all three problems: destroy Mondas using the ultra-powerful Z-bomb.

Both Dyson and Dr. Barclay are horrified by this plan. Dyson especially fears a backlash radiation effect on Earth itself. Barclay doubts Cutler can even get permission to use the bomb, but the General says he can and he will. As he waits for a connection to Geneva, he tells Ben that their Z-bomb is one of only three in the world, a doomsday weapon capable of splitting Mondas in half. Ben is as disgusted at the others.

Cutler gets through to Wigner on the microlink and tells him of the impending return of the Cybermen. Wigner has already been apprised of the spaceship formation as well as an increase in Earth's energy drain. He tells Cutler that he must hold on for now against the Cybermen. The General requests permission to use the Z-bomb, but Wigner says it is out of the question as they have no way of knowing the effects of the Z-bomb on Earth. Cutler then changes tack and asks if he is authorised to take "any action necessary" against the Cybermen themselves. Wigner doesn't realize what really is being asked and so agrees. They both disconnect.

Cutler takes this permission as including the Z-bomb and tells his staff to prepare for launch. All realize the trick he has played. Barclay and Ben both protest this plan. Cutler turns angrily on Ben, ordering a guard to take him away and lock him up with the Doctor.

Ben tries desperately to get through to General Cutler, telling him the Doctor said Mondas is actually in greater danger from this energy drain than Earth is. Eventually, it will absorb too much energy and burn itself out. All they really have to do is hold out and wait. Cutler doesn't believe a word of this and jokingly says he's only going to "accelerate" the destruction of Mondas. Barclay tries desperately to explain the potential radiation danger the Z-bomb poses to Earth, especially to Zeus 5 in orbit. But Cutler is resolute: they will detonate the bomb only when the capsule is on the other side of Earth and shielded. Whatever other risks there may be do not concern him.

Ben is grabbed by a guard, and only has a moment to encourage Polly to stay put and work on Barclay. Perhaps she can turn the nervous scientists over to their side completely while Ben looks after the Doctor. He is hauled out of the room.

The General is now ready to get on with the preparation of the Z-bomb. Barclay reminds him that both he and Dyson must be present to arm the bomb, so Cutler orders Dyson to come with him. Polly asks if she can be of help and volunteers to make coffee. Cutler agrees. As he leaves, he orders his staff to keep track of the capsule and the formation and to let him know the moment a Cyberman attack is imminent. Barclay watches them go, worry creasing his face.

In the Doctor's cabin, Ben tries in vain to wake him up. He then paces the room, looking for a way out. He produces a Swiss army knife and tries to pick the door lock, but it is technology far beyond that of 1966 and the knife does not work. Pacing some more, his gaze settles on a large grille set in the wall over the top bunk. He climbs up and peers in, seeing a dark airshaft, leading who knows where. He attacks it with his knife.

Dyson and Cutler join two technicians on a gantry surrounding part of the massive Z-bomb. All are wearing protective radiation suits as they work. Cutler is impatient to finish and gets irritated when Dyson seems to be stalling, but Dyson says there are routine checks which much be completed before fusing can begin. The General is cockily confident that the Z-bomb will solve all their problems, but Dyson isn't fully convinced. The radiation backlash could be devastating, and if the Doctor is right, all they really have to do is wait. Cutler chides him, knowing he will obey orders despite his misplaced misgivings, and tries to get him to admit he's scared. Cutler says he's only scared for his son but the Z-bomb is the only way to ensure that he will return safely. Dyson completes his checks and the fusing sequence begins.

Back in the Tracking Room, Barclay has been unable to contact Zeus 5. He instructs the R/T technician to keep trying. Polly brings him a cup of coffee and tries to engage him in conversation. The nervous scientists snaps at her to keep her nose out and then immediately apologizes. Taking the opening, Polly asks him about the possible radiation effects on Earth from the destruction of Mondas. At first, Barclay tries to sound nonchalant, saying no one really knows how much damage - if at all - there could be. But soon his nerve breaks and he admits that there will likely be some loss of life and some long-term damage to the environment. Polly asks if he is content to sit still and let this happen. Barclay obviously wishes he could do something, but feels powerless in the face of General Cutler's ruthless determination.

The radar technician announces 13 minutes to countdown and Polly gets desperate. She reiterates the Doctor's plan to simply wait for Mondas to burn itself out, acknowledging that General Cutler would probably perish in space if they do so, but that is only one life against possibly millions and they must take that chance. Barclay agrees, but he still dares not go against Cutler. Hopefully, Polly suggests sabotage - disabling the rocket in some undetectable way whilst pretending to go along with Cutler's orders.

However, the General returns and puts a swift end to the conversation. He does not notice their guilty looks as he is more interested in speaking to his son. There still has been no contact. What's worse, the formation of spaceships has left orbit and is heading on course to the South Pole. The General rejects a missile attack on the incoming ships and instead plans to ambush the Cybermen on the ground, using their own weapons. He orders a security detail outside under camouflage and issued with the captured cyber weapons. There is only 10 minutes to countdown. If they can hold off the Cybermen for that long, then the launch will be completed and the Z-bomb will do their work for them. Cutler receives word that the bomb is now in position on the rocket and ready for final checks. He just has time to do it before the Cybermen arrive and he heads out.

Polly takes the opportunity to grab Barclay, assuming his agreement to her plan, and drag him off. They must free Ben to help them.

They nearly frighten Ben to death when they burst into the cabin. Polly inquires quickly after the Doctor. The base physician could find nothing wrong, but he is still unconscious. Polly tells Ben their plan. Barclay says the rocket can be sabotaged very simply and untraceably but they must find a way to sneak into the rocket silo to do it. He points out the air shaft Ben was trying to break into, saying it leads to the silo. Barclay himself agrees to distract the guard and the technician on duty to give Ben time to get inside and go to a panel marked Plug Servo Leads. Barclay draws a picture and directs him where to find it. All he needs to do is pull out any one of the plugs and snip off a pin. The countdown will proceed as normal, but the fuel pump pressure on the rocket will fall to zero at blastoff and the rocket will not launch.

A klaxon blares through the base - the Cybermen have arrived. Barclay hurriedly gives Ben directions through the airshaft. They haven't much time.

Outside, a single Cyberman spaceship lands. Near the trap door to the base, guards are hidden in the snow with only the fronts of their cyber weapons exposed. A column of the creatures marches through the blowing snow toward the trap door. The guns fire and one by one the Cybermen fall.

During the distraction, Ben makes his way through the airshaft to the rocket silo. He reaches the end and looks down into the silo room. There is a technician running through some checks. Ben watches as a guard enters and gestures the technician out into the corridor. There, the two consult with a nervous-looking Dr. Barclay. Alone, Ben sets to work opening the grille.

In cabin 7, Polly tries again to wake the sleeping Doctor. She is still very concerned for his health. Suddenly, she hears someone unlocking the cabin door and scrambles to the top bunk, covering herself with the blanket and stifling a sneeze. A guard enters and checks that the Doctor and "Ben" are there. He leaves without further investigation. Polly uncovers and looks toward the grille, concerned for Ben.

In the silo room, Ben forces the grille open with a loud noise. Fortunately, no one is around to hear it and he scrambles down a nearby ladder and onto the elevated walkway. He hurries to the rocket panel to find the Plug Servo Leads.

In the Tracking Room, General Cutler watches a visual feed from outside, satisfied with the deaths of the Cybermen. He orders his men to capture the additional weapons and bring them down to the guard room, then turns to consult Dr. Barclay. However, Barclay is not in the room and no one knows where he is. Dyson says he's been gone since before he himself returned from the fusing room. Dyson suggests the rocket silo and Cutler's suspicions are raised. He hurries out of the room while Dyson gets on with his work.

Cutler finds Barclay in the corridor outside the rocket silo, still talking with the technician and the guard. He is at a loss to explain why he is here when Cutler confronts him and the General doesn't waste a second listening to his fearful stammer. He forces his way past and into the silo room, seeing Ben buried up to his shoulders in the servo panel. He barges up the stairs and onto the walkway, yanking Ben out by his collar, then striking the surprised youth. Ben flips helplessly backward over the walkway railing and onto the floor below.

His anger flaring, Cutler orders the guard to drag the unconscious Ben back to the Tracking Room. Then he orders the technician to check and see if Ben has done any actual damage. Barclay lamely offers to explain, but Cutler won't listen. There's work to be done and there'll be time for explanations later.

A short time later, everyone is back in the Tracking Room. Ben is slumped on the floor, unconscious, as Polly looks after him. Dr. Barclay is sitting at his console looking frazzled and fearful as Cutler completes a fiery tirade. He wants them all to know that if the rocket doesn't take off flawlessly, and his son is in danger, he promises to take the law into his own hands. Everyone knows what this means.

Cutler orders the countdown started, but Barclay tells him the preliminary checks are incomplete. Cutler shoots him a warning glance. He then turns his attention to the communications console and tries to reach his son. Lt. Cutler comes through clearly on the first try. He reports still no sign of the Cybermen spaceships, then asks when he's going to be able to return. The General tells him they must "deal with Mondas" first, but says nothing more. Terry reports some sluggishness on his capsule's controls and some fluctuation in power levels. Cutler reassures him they will get him down just as soon as possible.

Barclay announces all systems are go and gets a check-in from all of the launch control teams. Barclay starts the countdown at T-minus 200.

Polly succeeds in bringing Ben around, but his head is splitting and his memory is hazy. He cannot remember if he actually managed to sabotage the plug pins or not. He is not helped by the constant chatter and mounting chaos in the Tracking Room as the countdown continues.

At T-minus 135, a fault is announced on the range computers and Barclay stops the countdown. Cutler is on him in a second. Barclay says it is a minor fault, but Polly is sure they have found Ben's sabotage. The fault is cleared quickly and the countdown resumes. Polly is beside herself, certain that they have failed.

At T-minus 125, the Demeter rocket is raised from its silo to the surface. At T-minus 105, all personnel are cleared from the silo and the land lines and gantry retract. At T-minus 50, firing circuits go to auto action and timing goes to automatic.

In the Tracking Room, Barclay goes through his routine with automatic efficiency, all dissent momentarily forgotten. All faces in the room are set and hard, each person lost in his or her own thoughts as the tension mounts.

The rocket's engines fire, but as predicted, the fuel pressure falls to zero and the engines stop. The rocket remains on its launch pad. General Cutler, already tightly-wound, becomes enraged. He speaks in a menacing whisper, ready to snap. Polly voices her relief at their victory and Cutler turns on her, threatening the lives of Polly and her friends for their sabotage. He wants the Doctor brought to the Tracking Room, no matter how ill he is. But the Doctor has already regained consciousness and has brought himself here. He looks and sounds very weak, but is aware of what has happened and gloats over their victory. Cutler grabs the Doctor and shoves him over with the others. He will deal with them later and orders Dyson to regain contact with his son.

Polly is alarmed over how weak the Doctor looks. He is not sure what is happening to him, but blames it on "an outside influence". He guesses that his old body is "wearing a bit thin". This worries Polly even more, but the Doctor is more concerned with the problem at hand. He moves over to listen to General Cutler.

The General has turned on Barclay, ordering him to repair his sabotage and launch the Z-bomb. If not, he vows to kill Barclay as well as the Doctor and his friends. But Barclay is defiant, almost daring Cutler to try and launch the rocket. Cutler grabs him and shoves him back with the others. He starts to move toward them menacingly when a signal comes through from Zeus 5 - weak but audible. The General turns quickly to the communications console.

Lt. Cutler tells his father that his capsule is tumbling almost out of control, but there there is no panic in his voice. He hurries to impart his vital observation of Mondas - it is brightening and darkening alternatively, brilliant one second then dark the next. The Doctor hears this and knows his prediction was accurate. Mondas is almost at its limit of energy absorption and will burn up soon. But Cutler doesn't even hear this, so intent is he on his son's plight. He even silences the radar technician when he reports Cyberman spaceships approaching Earth in force again.

All the General cares about is his son. He listens and watches helplessly as Terry reports the total loss of capsule control. Then the screen goes black and contact is lost. He stares for a moment in disbelief and then the General loses control. He orders Dyson to get the signal back again, completely ignoring the ongoing reports of a Cyberman spaceship landing nearby. But when Barclay dares to comment on this, Cutler hears and turns on them all. In his mind, the saboteurs are the only enemy. They are responsible for the death of his son. They will be responsible for the destruction of Earth. They are the enemy and they must die.

He pulls out a pistol and aims it at the group, trapped against the back wall. The technicians continue reporting, announcing the landing of the ship. There is no defense this time, but Cutler, out of his mind with anger and grief over the apparent death of his son, hears nothing and cares about nothing else. He calls the Doctor and his friends murderers. He points the gun at the Doctor and prepares to execute them all one by one.

Just at that moment, pandemonium breaks out in the hallway outside. There is muffled gunfire followed by the unmistakable whine of a cyber weapon. The Cybermen have again invaded Snowcap base. The General turns to the doorway as a Cyberman appears. He comes back to himself for a brief second, firing at the creature, but he only gets off one harmless shot before he himself is gunned down by the Cyberman. He collapses in a smoking heap while the Cybermen enter the Tracking Room in a stunned silence.

The Doctor thanks the lead Cyberman for saving their lives but is ignored. The Cybermen disarm all the soldiers and herd the staff together in one corner. Ben wonders ironically if they'll get any gratitude for saving Mondas, but the leader says they have detected the missile aimed at the planet and do not consider it a threat. The Doctor assures him it could have done significant damage, but that they neutralized it and saved Mondas. In exchange, the Doctor boldly asks the Cybermen to abandon their plans to conquer the Earth and instead live in peace here with the humans when Mondas disintegrates.

The Cybermen confer on this point, appearing serious about his offer and leaving the Doctor and his friends alone for a moment. Ben and Barclay are certain the creatures cannot be trusted, but the Doctor says their only chance is to play for time. It is not necessary to trust them.

The Cybermen announce that they cannot negotiate while the Z-bomb is still aimed at Mondas and they wish it to be disarmed. The Doctor, consulting with Barclay, agrees to do this, but he is only trying to waste enough time for Mondas to burn up. They want the warhead removed and placed below ground level. Barclay suggests the Radiation Room, the deepest part of the base. To make certain the others comply, they will take Polly to their spaceship and hold the Doctor in the Tracking Room. Both are hostages.

Ben tries to protest, offering himself in Polly's place, but both the lead Cyberman and the Doctor dissuade him. He is needed to help with the warhead and the Doctor has his own plans to deal with the Cybermen. He extracts a promise from the Cybermen to return Polly once the warhead is safely stored away, and he sends her off with them, reminding her to take her coat against the cold.

Polly is taken to the Cyberman spaceship. It is as cold inside as the weather is outside. There is no need for heat. She is shown to some sort of chair with bars in front like a cage. She refuses to sit and the Cyberman zaps her with flash of light. She slumps unconscious into the chair. The Cyberman closes the cage and she is trapped.

In the Tracking Room, the Doctor is alone with two Cybermen. A video message comes through from International Space Command HQ - Secretary Wigner calling for General Cutler. It goes unanswered for some time, until the Cybermen indicate for the Doctor to answer it. He does so awkwardly, saying only that the General is unavailable. Wigner reports that there have been mass landings of Cybermen all over the world. Just as he says this, Cybermen burst into his own office. The Cybermen are now controllers of the Earth.

The Cyberman in Geneva orders his counterpart at Snowcap to proceed with their "second objective", but there must be time for the Cybermen to evacuate before it occurs. The Doctor knows immediately what this means - the destruction of Earth, using the Z-bomb itself. The Doctor grabs an internal microphone and gets a message to his friends in the Radiation Room to stop work immediately.

Ben, Barclay, Dyson and a technician called Haines have succeeded in bringing the Z-bomb warhead down off the rocket and into the Radiation Room. They hear the Doctor's warning and realize that they have been fooled into helping the Cybermen. Dyson immediately regrets his opposition to Cutler and believes that they've signed the death warrant for the entire planet. Ben and Barclay still believe they can prevail. They all try to look busy as a Cyberman passes by outside the door and checks on them through the window.

Ben has had a thought. He wonders why the Cybermen need humans to do this work. They are stronger and more advanced than humans and could likely have dismantled the Z-bomb quite easily on their own. Why, then, do they need human labour and why have the Cybermen not come into the Radiation Room to at least supervise? Barclay realizes that the creatures must have some aversion to radioactivity. Following the Doctor's command, Ben decides to test the theory and play for time.

All four men lie down on the floor as if injured. Ben shouts for help and draws the Cyberman guard to the door. The creature hesitates for a long moment before opening the door and entering. Shortly, the Cyberman slows to a stop and drops its weapon. Ben gets up and shoves it outside, closing the door. Dyson wonders why they didn't try to escape, and Ben tells him they have the Cybermen right where they want them. They cannot get inside due to their weakness to radioactivity and cannot set off the bomb. All there is to do is to wait for Mondas to burn up. But Ben is aware that the Cybermen still hold the Doctor and Polly.

The Doctor and the Cybermen have watched on the monitor. The Doctor is pleased with the stalemate. He offers again to negotiate, but the Cybermen are only interested in victory and will kill the Doctor and Polly to achieve it. The Doctor is quick to remind them that harming them will not save Mondas. The lead Cyberman addresses Ben and the others, telling them that one of their planets has to be destroyed for the other to survive. It shall be Earth that is destroyed and he offers to save them all by taking them back to Mondas. A nervous Dyson wonders if they should agree, but Ben of course refuses. He tells them they must free Polly and the Doctor if they want the humans' help after Mondas burns up. The Cyberman refuses to acknowledge the possibility of defeat and orders the Doctor taken out to the spaceship. The Doctor promises they will regret this.

The lead Cyberman addresses Ben and the others, giving them just 3 minutes to fuse the warhead or Polly and the Doctor will be eliminated.

In the Radiation Room, Ben watches the Doctor being dragged out of the Tracking Room. He fears for the safety of his friends and now doubts the wisdom of his plan. Predictably, Dyson wants to go along with the Cybermen. His nerve is shot and he will take any opportunity to survive. But Barclay urges them to sit tight and wait. Millions of lives are at stake, not just two. Ben, however, wants action and so attacks the monitor system, disabling it completely. They can't communicate with the Cybermen, but neither can the Cybermen spy on them. He begins to outline his new plan.

In the Cyberman spaceship, the Doctor has been imprisoned in one of the chair-cages next to Polly. His energy seems to be dangerously low as he slumps against the bars. Polly is concerned over the rising and falling mechanical whine she can hear in the ship. The lights in the room also go from bright to dim in time with the noise. The Doctor rouses somewhat, listening. He thinks that because the ship receives its energy from Mondas, it too will be affected when the planet absorbs too much energy. Perhaps the ship will even blow up, with them inside.

Ben has come up with a plan to attack the Cybermen using radioactivity. He wants to hold them off, stalemate them again but not kill them outright. But he needs something radioactive and portable. Barclay can think of nothing and Dyson's rising hysteria is becoming obstructive. Ben points out the nuclear reactor and Barclay thinks they could remove the reactor rods if all of them worked together. Again, Dyson refuses, believing that they can trust the Cybermen. Ben has had enough of this. He tries to convince him that the Cybermen are lying and that either way he'll be killed if he doesn't at least try. This seems to work and Dyson joins in.

In the Tracking Room, the Cybermen wait for the 3-minute limit to expire. On a monitor screen, they can see Mondas rotating rapidly, wildly alternating between bright and dark. The planet is nearing saturation point and they must act quickly if they are to save it. They try to activate the Radiation Room monitor but it does not work. They realize what has happened and head for the Radiation Room.

Ben and Haines are nearly finished removing the fuel rods from the nuclear reactor. Barclay is set to turn on the emergency power as soon as main power goes out. He worries that there will only be one hour's worth of light and heat, but Ben reminds him that if this doesn't work, the cold will be the least of his worries.

Once the rods are removed, Ben sends Dyson and Haines to opposite ends of the now-deserted corridor. Each one has a fuel rod. Ben grabs the cyber weapon and goes out to draw the Cybermen to them. As soon as they hear the weapon fire, that will be their cue to come forward with the fuel rods. Hopefully, the Cybermen will be trapped between them and incapacitated by the radiation.

But the Cybermen have a plan of their own. They carry with them a canister of lethal gas which they will use if necessary, although they hope to capture the humans alive. Ben sees them coming and races ahead back into the Radiation Room, where Barclay waits. The Cybermen still will not come inside and so demand an answer to their ultimatum through the door. Ben tells them they can come and get the bomb if that's what they want. Instead, they begin pouring gas into the room.

Ben realizes what is happening and adjusts. He grabs the weapon and races to the far side of the room and signals Barclay to open the door. He does so and joins Ben. Through the thickening gas cloud, Ben cannot see the Cybermen, but fires the weapon anyway. He is not trying to kill them, but to call Haines and Dyson forward. The two Cybermen pull the door closed in order to let the gas do its work. Ben and Barclay are trapped and the others seem to be taking their time.

However, Haines and Dyson do arrive from opposite ends of the corridor, trapping the Cybermen between them. They stagger and stumble, finally collapsing onto the floor. The two men hurry into the Radiation Room to pull out Ben and Barclay. All four head back to the Tracking Room.

Soon, the soldiers and technicians are back at work. The Cybermen seem to have left the base. Barclay orders the fuel rods replaced in the reactor as soon as possible. Ben is trying to figure out how to rescue the Doctor and Polly. He fears that the spaceship will return to Mondas and he will never see them again. Barclay finds one of the Cyberman communication devices discarded on the floor. Ben finds a way to activate it, hoping it will at least keep the Cybermen here.

It has the intended effect, but on a much larger scale. A troop of Cybermen returns to the base, just as the emergency power begins to fail. The humans hope that they can hold out using the captured cyber weapons, but the Cybermen continue relentlessly. Even in the chaos, Ben's gaze is drawn to a monitor screen showing a picture of Mondas. The planet now rotates crazily and the brightening and darkening cycle is much quicker. A haze of gas surrounds it. All eyes are soon on the planet as it appears to melt and disintegrate before them. The Doctor's prediction has come true.

Ben remembers the Cybermen outside and glances over. He sees that they, like their planet, are dying. Without power from Mondas, the creatures slow to a stop like toys winding down. They teeter and fall one by one. Then they too begin to disintegrate. All of the organic components break down without energy to sustain them, leaving behind only their metal and plastic parts amid smoking debris. On the monitor, Mondas is now nothing more than a cloud of gas and dust drifting harmlessly away from Earth.

In the aftermath, a miracle occurs. The voice of Terry Cutler crackles through from the radio. He is alive and wondering when the Snowcap technicians might have a moment to get him down. With the destruction of Mondas, all systems are fully operational and his capsule is functioning normally. Barclay takes charge, telling Lt. Cutler he'll have to stay put for the moment until things are sorted out at the base.

Barclay contacts ISC HQ just as Ben remembers that the Doctor and Polly are still trapped on the Cyberman spaceship. He barely listens as he suits up to go outside. Wigner tells Barclay that the Cyberman menace has ended simultaneously all over the world. He wants a full report from Snowcap as soon as possible. With so much to do, the base staff do not even notice as Ben leaves to rescue his friends.

In the ship, all the Cybermen are dead. The Doctor is unconscious in his cage, slumped against the bars. Polly is cold, alone, and frightened. The ship is deathly quiet when Ben enters. He frees a grateful Polly and tries to comfort her. Together they try to wake the Doctor, telling him that it's all over.

Slowly, very slowly, the Doctor regains consciousness. His speech his slow and indistinct. His eyes are glassy and his gaze far away. According to him it's far from being "all over". All the Doctor can think about is returning to the TARDIS immediately. He brushes off all offers of help except a scarf offered by Ben. Then he bolts out of the ship, leaving his companions alone. His last words to them: "Keep warm."

Then he is gone, moving faster than they thought he was capable. Ben and Polly take a moment to wonder what all this means and to spare a thought for the dead Cyberman at their feet. Then they must hurry after their friend.

They get outside only to find that the Doctor has already reached the TARDIS and the door is closed. They push through the blowing snow and pound on the doors to get the Doctor to open up.

Inside, the Doctor seems lost in his own world. The console room is darkened and the Doctor hunches over the controls as he operates them, leaning on the console for support. He appears to be dying. The last thing he does is open the doors to admit his companions. They burst inside to see the Doctor's thin body silhouetted by the light from the TARDIS walls.

He collapses onto the floor and Ben and Polly rush to help him. Polly pulls the scarf which has fallen across his face just as the sound of the TARDIS dematerialization begins. Suddenly, the Doctor's entire body is bathed in brilliant light from no discernable source. It reaches blinding brightness and then fades.

When Ben and Polly open their eyes again, the face and body left behind on the TARDIS floor is that of an entirely different person...