The android flagship Antares and its warship escort, the Pentagon, have travelled to the planet Telos to stop the Cybermen from activating their master hibernation vault... but they arrive to find only a dense electromagnetic field and a cluster of asteroids. Telos was in a particularly vulnerable position, cosmically speaking, and at some point in the past decade, the planet must have been destroyed by a random asteroid strike. The Cybermen presumably predicted and planned for this possibility; though nothing could survive a direct impact, the vault was likely built to survive the planet breaking up around it. If intact, the vault must be somewhere amongst the debris, but the electromagnetic interference makes long-range scans impossible, so it's going to be a long, slow search... and the Cybermen could be out there, searching for the vault themselves, right now. Liam Barnaby thus finds himself at a loose end; the androids were built to last, but he wasn't, and it's entirely possible that he could die of old age while the androids continue their mission to save humanity. At least he's been given guest quarters, thanks to Samantha's support, but he recognises that Riordan is jealous of the attention Sam is paying to him. The differences between humans and androids suddenly don't seem so significant to Liam, and although Sam feels that she's accomplished something by showing Liam the error of his ways, he's not so sure that changing a single person's viewpoint will be enough. And he's afraid of what might be happening back on Earth...

Back on Earth, Paul Hunt has been appointed as emergency executive-in-chief in Karen Brett's absence, due to the unfortunate "unavailability" of the vice-president. Cybermen are patrolling all of Earth's key cities, and the rate of conversion is gradually being increased; by the time the authorities realise what's really happening, it will be too late. Now the Cyber-Planner need only wait for word that Brett's mission has succeeded...

Liam visits the bridge of the Antares, but Riordan contemptuously dismisses his offer of help; unlike the androids, Liam requires sleep and rest, and if his concentration lapses at a critical moment, they could miss what they're looking for. Liam points out that this means there's no reason for him to be on this mission -- so why did Riordan let him come along? Is it just because Sam asked for him? Riordan storms off the bridge, but Liam follows him and provokes Riordan into lashing out by reminding him of how Samantha brought Liam back to his senses back on the freighter. Liam doesn't defend himself when Riordan attacks him, and slowly Riordan comes to realise that Liam is making a good point: Riordan is jealous of Sam's possible feelings for Liam, and Liam, a human being, actually understands this and is trying to have a conversation with him about it. One human has finally accepted that androids are his emotional equals; only a few billion humans to go, and the war could be over.

The conversation is interrupted when the Antares is shaken by a nearby explosion, which the androids soon determine was caused by the destruction of the Pentagon. Something passes across the Antares' flight path, and Liam recognises the energy readings of the XP-900, an experimental warship that must have been pressed into service by the Cybermen. The androids detect the XP-900 coming straight for them and have no choice but to power up their own weapons, though this may mean giving away their position -- but before they can engage in combat, both ships are struck and damaged by a particularly large fragment of planetary debris. The Antares is sent spinning out of control, venting fuel, straight towards a large object... a large metallic object. The androids have found the master hibernation vault, but now they can't stop themselves from colliding with it. Meanwhile, the Cybermen manage to regain control of their ship, as the Cyber-Leader who was once Karen Brett dismisses the vague memory of fear in her mind. There is nothing to fear; they have detected the trail of fuel vented from the second android ship, and as soon as it's within range, they will destroy it and proceed to activate the master vault...

The Antares survives its collision with the vault, possibly just because they couldn't have come all this way to fail at the last moment. Liam and Samantha take a reconnaissance team into the vault while the others repair the ship, and inside, looking at the thousands of Cybermen in suspended animation, Liam can no longer remember why he used to distinguish between humans and androids. The Cybermen destroyed their humanity to achieve immortality, and if that's the future of the human race, than all of humanity's past achievements will count for nothing. The androids plan to repair their ship, get out and destroy the vault with long-range missiles -- but that plan has to change when Liam and Samantha realise that the Cybermen are starting to move. The collision damaged the vault's hibernation circuitry, and the army inside is waking. With time running out, the androids' only option is to plant explosives throughout the vault -- and they most likely won't have time to get away and trigger them by remote control. Nevertheless, Riordan has little choice but to give this revised plan the go-ahead, even though it's Liam who suggested it.

While looking for places to set the charges, the androids find a communications centre; this is an important target, as destroying it should prevent the Cybermen from sending the signal to activate the other hibernation vaults. The XP-900 then enters sensor range, heading straight for the android ship. Liam suggests stalling them by contacting them and pointing out that destroying the Antares will cause damage to the vault. Riordan is still reluctant to take advice from a human, but is forced to do so when the Cybermen launch a missile. He contacts the Cybermen and warns them that the Antares is embedded in the master hibernation vault, and the Cybermen abort the missile. The Cyber-Leader then orders Riordan to accept the inevitable and surrender, and Liam is shocked to recognise the Leader's voice, underneath the modulation, as that of his old friend Karen Brett. As Liam hesitates, shaken, the doors of the XP-900 open up and a thousand Cybermen emerge, spacewalking towards the android ship...

Liam pulls himself together and tells Riordan to let him speak to Brett, and again, Riordan has little choice but to do so. The Cyber-Leader demands to know why a human is collaborating with androids, but Liam questions how she knows he's a human. It must be because she recognised him; deep down, some part of her must still be Karen Brett. He begs his old friend to call off her Cybermen, but she refuses, insisting that there is no Karen Brett. Liam warns her that they've got the vault rigged to explode and aren't afraid to detonate their charges, but the Cyber-Leader concludes that, since humans have an emotional attachment to life and androids wish to emulate humans, they will not risk destroying themselves. She cuts off the transmission, and Riordan orders Commander Brinner to take charge of repairs while he leads a squad of androids out to face the Cybermen. She refuses, insisting that their people need their leader to survive, and he's reluctantly forced to let her defend the ship in his place. Whatever the results of the battle, however, it's likely that the androids won't have time to retreat before they must detonate the explosive charges in the vault. Meanwhile, aboard the XP-900, the Cyber-Leader struggles to resist the strange memories surfacing in her mind, of her parents' pride and of her friendship with Liam Barnaby. The androids must be destroyed, the charges disabled, the vault activated. There is no Karen Brett.

Liam places the charges in the communications centre, Samantha places the charges in the reactor, and another android places the charges in the hibernation chamber. With time running out, Spavin sends a test signal from the bridge of the Antares, but the androids in the hibernation chamber fail to respond -- and Liam and Samantha hear clanging sounds from the hibernation chamber, indicating that the Cybermen have fully woken. Sam calls back to the Antares, ordering them to blow the charges immediately, but gets no response; the Cybermen have swept through Brinner and the other defenders and have boarded the android ship, destroyed the detonation control and killed Spavin.

Liam and Sam, apparently the only survivors, now have no choice but to detonate the bombs manually -- but there are only two of them, and three targets that must be destroyed if they are to prevent the Cybermen from surviving to signal the other hibernation vaults. Fortunately, Riordan catches up with them, having escaped from the Antares just in time. Liam will try to get to the bombs in the hibernation chamber, Riordan will take the transmission chamber, and Samantha will take the reactor. Unfortunately, even if they succeed, nobody will ever know that androids and humans managed to put aside their differences for a common cause. Meanwhile, the Cyber-Leader sends a report back to Earth: Telos has been destroyed as predicted, but the vault has been located, the androids have been neutralised, the activation signal will soon be sent, and there is no Karen Brett. On Earth, the Cyber-Planner receives the report, but dismisses the bizarre final statement as unimportant and waits for word of the Cyber-army's awakening. Nevertheless, Paul Hunt is a little concerned...

The Cybermen in the hibernation chamber find the explosives before Liam can get to them, and he is forced to flee as they disarm them. Riordan is attacked by the Cyber-Leader before he can detonate the explosives in the transmission chamber, but he manages to trigger the bomb, which explodes in the Cyber-Leader's hands. Liam joins Sam in the reactor, but her bomb fails to explode; the Cybermen must have found it and disarmed it. Riordan, dying, contacts Sam and warns them that the Cyber-Leader survived the explosion; although mortally injured, and obsessively repeating that there is no Karen Brett, the Leader manages to activate the transmitter and kills Riordan as he warns Sam and Liam that the reactor must be shut down at all costs. Liam looks for shelter, but the radiation shielding is too flimsy to protect him and Sam from gunfire... but it then occurs to Sam that the Cybermen must need radiation shielding because they are vulnerable to radiation poisoning. Androids aren't, so Sam shoots out the shielding and raises the fuel rods, telling Liam to seal his spacesuit. The Cybermen break into the reactor moments later, but collapse before they reach Sam.

Liam also falls victim to radiation poisoning, but Sam carries him back to the Antares, where Spavin had been keeping a supply of anti-radiation drugs. The dazed Liam is vaguely aware of Cybermen collapsing dead around them as the radiation sweeps through the vault. With the reactor powerless, the transmission signal was cut off, and Sam and Liam can only hope that they succeeded before it reached any of the other vaults throughout the galaxy. As Sam sets course away from the vault, the Cyber-Leader suddenly bursts onto the bridge, demanding that they return. However, she has also been affected by the radiation, and she haemorrhages neural dampening fluid and collapses before she can attack her enemies. Liam points out that it's only bloody-mindedness keeping her going, and that's an emotion. The Leader tries to insist that there is no Karen Brett, but Liam won't accept that; dying, she can remember her parents, and that hurts her. Despite everything that she's done and that's been done to her, Liam will still mourn Karen Brett when she dies, and he thus demands that his old friend answer one question. Even if the Antares is too badly damaged to get him and Sam back to Earth, they have proven to themselves that humans and androids can work together, and perhaps that's enough to overcome the sterile, dead future that the Cybermen have promised. Or perhaps not. Who does the future belong to? Man or Cyberman? Dying, Brett begins to say "Cyber..." -- but when she finishes with "Man," it's in her own voice.