About a mile beneath the surface of the planet Bortresoye, a woman named Excelsior is practising her state-of-the-union address, but she’s really more concerned with her personal appearance than with the text of her speech. As Excelsior’s makeup assistant adjusts her wig and touches up the skin around her jaw line, her Ministers for War and for Peace, Voss and Tralfinial, advise her to give her people adequate warning about the danger that they’re in. She upbraids them for their caution, insisting that there is no doubt that the war can be won -- even if the government has retreated to a protective bunker as a precaution. Voss and Tralfinial are far too terrified of Excelsior to tell her the truth about conditions on the surface, and say nothing as she addresses her people via public broadcast, sadly informing them that she has authorised the use of pre-emptive force to drive back the enemies massing on their borders. However, her jaw line splits during her address, and once she’s finished, she orders that her hapless makeup assistant be executed.

The Doctor is separated from Charley and C’rizz in the interzone, and encounters the shades of his dead companions Katarina and Adric. He soon realises that these are illusions generated by the Kro’ka, and is furious with his tormentor for digging up such painful memories. However, the Kro’ka suggests that the difference between life and death may not be as clear-cut as the Doctor believes. The Kro’ka also advises the Doctor to consider the nature of his friends’ faith in him. His next question will involve fundamental questions of faith, life and death, and according to the Kro’ka, all things must die, and the end of one journey is the beginning of another. The Doctor emerges from the interzone in the ruins of a dead city, where Charley and C’rizz are waiting for him. As they shelter from the freezing wind in a ruined building, the Doctor identifies the destruction as the result of atomic warfare, and warns his companions that any survivors of the initial explosion will doubtless have died from radioactive fallout or exposure to the conditions of nuclear winter. But the Doctor and his companions will be stuck in this wasteland until they find out what the Kro’ka expects them to accomplish here...

In the bunker, Voss and Tralfinial try to explain the situation to Excelsior without admitting their own culpability. All Excelsior really cares about is her forthcoming victory parade, which Voss and Tralfinial advise her to postpone until normality has been restored to the surface. To their irritation, their meeting is then interrupted by Excelsior’s most trusted advisor, Landscar, and since it’s clear that she prefers his advice to theirs, they storm out. Privately, they’re aware that the war killed off most of the planet’s population and rendered the surface uninhabitable, but they’re far too frightened to admit this to Excelsior... and neither really understands who Landscar is, or what he’s doing in the bunker.

Meanwhile, Landscar is trying in his own way to convince Excelsior to see the surface of the planet for herself, claiming that there is a way for her to do so without actually setting foot outside the bunker. At Landscar’s urging, Excelsior opens her mind to the natural forces of the planet, and feels herself becoming at one with them. Voss and Tralfinial investigate the ensuing commotion to find that the meeting chamber has been torn apart, as if a storm has swept through the room. Excelsior and Landscar insist that everything is under control, and the Ministers retreat, assuming that Excelsior lost her temper with Landscar. But before Landscar can resume preparing Excelsior for the changes to come, he senses something odd; there is new life in the world outside...

On the surface, the temperature has dropped, the wind has picked up, and Charley and C’rizz are rapidly losing all hope of survival. As the Doctor tries to revive their optimism, the wall caves in, nearly crushing them. The Doctor suggests moving on, but C’rizz wants to wait for the wind to die down; however, the Doctor informs him that a nuclear winter could last for centuries. The argument becomes moot when the roof collapses, burying them all beneath the rubble. C’rizz is protected by his exoskeleton, but Charley is pinned to the floor by a collapsed beam and the Doctor is completely buried. C’rizz shifts the beam that had pinned Charley, but she finds that she is still unable to move. C’rizz insists that she’s just in shock, and leaves to find help. Soon after he’s gone, however, Charley hears the sound of whispers, as if she’s surrounded by an invisible crowd. Someone eventually does show up in the ruins, but it isn’t C’rizz...

Out in the wastes, C’rizz also hears the sound of whispers on the wind. He then encounters a strange man who simply stares at him and repeats the words that C’rizz is saying to him. C’rizz tries to explain that his friends are trapped and need help, and the man eventually follows him back to the ruined building. On the way, C’rizz tries to learn more about what happened here, but his new companion seems unwilling or unable to answer. When they reach the collapsed building, C’rizz has learned only the man’s name, Requiem -- and when he enters the ruins, he finds that the Doctor and Charley have vanished...

C’rizz finds evidence that someone removed the Doctor and Charley from the building, but there’s no way of knowing where they are now. The storm is growing worse outside, but Requiem, who seems to have grown comfortable enough to speak to C’rizz in full sentences, offers to show him to another, more stable shelter. They near the shelter, but are forced to start running when it begins to rain -- acid rain, which burns C’rizz’s skin wherever it touches, and yet, oddly, doesn’t seem to bother Requiem.

Thanks to Landscar’s guidance, Excelsior is becoming at one with the natural elements of her world; she can feel the acid rain dripping from her fingertips, and can sense a subterranean river raging past the bunker, threatening to burst through the walls. However, despite her sharp words to Voss and Tralfinial, she is also concerned about Landscar’s decision to allow two strangers into the bunker. She scoffs at his claim that the newcomers are from another world, but Landscar has his own reasons for believing this to be true. Irritated by his cryptic comments, Excelsior reminds him that she can still have him executed if he begins to displease her. She then changes the subject to the mood of her people as they prepare for the victory parade, and Landscar assures her that her people are indeed eagerly awaiting their reunion with their leader...

The Doctor tries to convince Charley not to give up hope of walking again, but she insists that she’s just being realistic, and his refusal to accept her condition is beginning to grate on her. Voss and Tralfinial then arrive, and Charley’s nurse retreats reluctantly, allowing the Ministers to question the newcomers in private. The Doctor soon determines that the ministers are too intelligent to believe any cover story, and thus admits the truth: he and Charley are innocent travellers, sent through the interzone barrier to this world by a creature called the Kro’ka. This means nothing to Voss or Tralfinial, but the Doctor does learn something significant from them; this is the only survival shelter on the planet, and the few dozen people within are presumably the only survivors of the nuclear war.

Requiem has taken C’rizz to the city archives, where he worked before the war. C’rizz listens sympathetically to Requiem’s description of the devastating attack, which reminds him of the arrival of the Kromon on his homeworld and the loss of his family. The only reminder he has of his lost loved ones is a half-moonstone his father gave him on his wedding day, keeping the other half so he and his son would always have a reminder of one another. Requiem has nothing to hold onto, and doesn’t really understand what keeps him going; worse, his people were destroyed not by alien invaders, but by themselves. It seems as though Bortresoye has always been at war; when Requiem’s leaders launched the final attack, their planet’s resources had already been exhausted and all other living species on the planet eradicated through excessive harvesting or scientific experimentation. This war has been, quite literally, the end of the world.

Voss begins to question the Doctor about his mode of transport, and when the shelter is shaken by an earth tremor, he admits that he intends to abandon his doomed world and save himself. The disgusted Tralfinial reminds Voss that he is just as responsible for the war as any of them, but their argument is interrupted when Excelsior sweeps grandly into the hospital to speak with the Doctor and Charley, whom she has apparently decided to regard as war heroes. Charley politely comments on Excelsior’s victory gown and suggests some much-needed improvements, to Excelsior’s delight; however, Excelsior then begins to deliver a stirring speech about those who died in the war to make their world a better place to live in, and the Doctor realises that she doesn’t understand the true conditions on the surface. Excelsior dismisses her ministers and questions the Doctor further, but she doesn’t seem to understand his apologetic, veiled hints that the victory parade might not be as well-attended as she hopes. Eventually, Charley gives in and just flat-out tells Excelsior the truth, but Excelsior dismisses her talk as rubbish... and as dangerously bad for morale. If they continue to speak in this vein, Excelsior will have them silenced...

Before she can carry out her threat, a messenger arrives with urgent news: the outer wall of the shelter has been breached by the constant earth tremors, and the subterranean river is about to flood the lower levels. Voss urges her to evacuate, but she refuses to let him push her into giving an order before she’s ready to do so herself. She eventually does give the order, but too late; the inner wall has fractured and the bunker is being flooded. The remarkably unconcerned Landscar tells the Doctor that all things must die, but the Doctor doesn’t intend to stand around waiting for that to happen, and he rushes back to the hospital to rescue Charley as Excelsior and her advisors head for the lift. The nurse helps the Doctor to get Charley onto a stretcher and wheel her to the lift, but the collapse of the bunker has destroyed the nurse’s last hope. Voss has returned in the lift, but the floodwaters catch up to them and sweep the nurse away just as the Doctor gets Charley inside. Voss closes the lift doors and ascends, as the frustrated Charley demands to know why they’re saving her useless life over that of the nurse. The Doctor insists that Charley has just as much right to survive as any of them -- but Voss has his own reasons for wanting to keep the newcomers alive...

On the surface, C’rizz learns more about Excelsior and the war, the last and greatest mistake of the people of Bortresoye -- and one that nobody will live to learn from. C’rizz then hears whispering from the shadows, and Requiem admits that there are about fifty or so other survivors in the remains of the archive. C’rizz, delighted, tries to convince Requiem that this means there’s still hope for the future; as long as there are survivors, they can rebuild their community and try to make things better. Requiem also admits that there is a government bunker three miles away, possibly containing food and medical supplies, and C’rizz urges him to get his people moving; even if the bunker contains those who started the war, they will have to face their responsibilities and care for the survivors of the holocaust they created.

The lower levels of the bunker have been completely flooded, however, and when the Doctor arrives in the upper levels, he discovers that he, Voss, Charley, Excelsior and Tralfinial are the only survivors. Over a hundred people were killed in the flood, but Excelsior is more concerned with the damage to her gown, and threatens to have Voss executed for failing to rescue her seamstress. Fortunately, Charley manages to convince Excelsior to be benevolent for the sake of her few remaining subjects, and Excelsior magnanimously agrees to grant a stay of execution. As she sweeps off to prepare for her victory parade, the Doctor and Charley agree that Excelsior been driven completely insane by the consequences of her declaration of war.

As the Doctor, Charley and Voss try to calm the increasingly panic-stricken Tralfinial, C’rizz tries to understand what happened to the people who supposedly accompanied him to the shelter. For a moment, he thinks he hears Requiem speaking to him, but none of the others can hear him. The Doctor suggests that C’rizz may be hallucinating due to exposure, but C’rizz points out that he somehow found out that this bunker was here, and insists that it’s because Requiem told him about it. Voss seals the bunker again, but now Tralfinial can hear accusing voices surrounding him, and he flees in a panic, insisting that C’rizz has brought evil spirits into the bunker with him. Before following his fellow minister, Voss pauses to ask the Doctor one question: can he pilot a rocketship? The Doctor cautiously answers in the affirmative, and Voss, satisfied, sets off after Tralfinial.

Tralfinial is telling Excelsior that Voss has deliberately opened the shelter and exposed them to the evil spirits that haunt the wasteland outside. Voss tells his leader that Tralfinial is talking nonsense, and Tralfinial, pushed too far, pulls a gun on Voss and tells Excelsior the truth about the war and the conditions outside. Excelsior chastises Tralfinial for threatening violence in her presence and orders him to surrender his gun. He calms down somewhat and does as she asks, but when he then tells her that the planet has been damaged beyond repair, she shoots and kills him, telling the startled Voss that his ravings would only have hurt morale. She intends to tell her people that Tralfinial gave his life for the war effort. Unnerved, Voss takes his leave of Excelsior, who settles down to practice her speech for the victory parade. The voices of the dead circle about her, including that of Requiem, but she ignores them; she knows what they have to say to her, and doesn’t care.

Meanwhile, C’rizz has told the Doctor and Charley of his conversation with Requiem. If it really happened, and if what Requiem said is true, then the planet is truly beyond hope of recovery, and they have no choice but to abandon it, even if this means leaving the TARDIS behind. Rather than give up all hope of recovering his ship, however, the Doctor chooses to interpret the Kro’ka’s last cryptic remarks to mean that the TARDIS has already been moved off-world. Requiem’s identity still remains a mystery, and the Doctor suggests that perhaps C’rizz found shelter in the city archives, fell asleep while reading, and hallucinated the existence of a friend in a delirium brought on by exposure. The Doctor admits that the Kro’ka showed him images of the dead before he entered this zone, but he doubts that the Kro’ka is responsible for the destruction of this world; he is simply an observer, and the blame for the war lies entirely with this zone’s inhabitants.

Voss arrives, and is forced to admit that Tralfinial won’t be accompanying them. Disgusted by the people’s foolishness and Voss’ selfish desire to save his life, the Doctor nevertheless agrees to accompany Voss to the rocket pad to check out their only means of escape, a rocket intended for space travel but then converted to serve as a weapon of war. C’rizz stays behind with Charley, and tries to assure her that they’ll leave this desolate world behind and find a place where Charley can receive medical care. However, Charley knows that she’ll only slow down her friends, and asks C’rizz to do the right thing -- as he did with L’da -- if it becomes necessary to abandon her so he can escape. Angered by her fatalism, C’rizz insists that she will be coming with them, whatever happens. Excelsior then arrives, and when C’rizz realises that this is the women who launched the war, he lashes out at her in a rage, threatening to throw her out of the bunker to face her just fate in the wilderness outside. Amused by his forthrightness, Excelsior offers to make him her new minister for peace, and the furious C’rizz storms off to clear his head. As he walks away, he hears the voice of Requiem calling to him, and fails to hear Charley asking him not to leave her alone with Excelsior...

Excelsior realises that Charley doesn’t like her much either, but insists that she doesn’t care about popularity; all she cares about is making the world safe for her people. When the irritated Charley makes a snide comment about the loss of future generations, however, she learns from Excelsior’s puzzled response that the people of Bortresoye do not reproduce. Excelsior claims that her people have reached their evolutionary pinnacle and have no need to change, but when she chastises Charley for daring to judge the people of Bortresoye, Charley realises that Excelsior is fully aware that she and the Doctor are not natives. And since there’s no reason for her to have reached this conclusion, she must have known all along that there are in fact no other survivors. She’s not as delusional as she’d like the others to believe. However, Charley realises too late that she shouldn’t have revealed she knows this. Paralysed, she is unable to struggle as Excelsior smothers her with her own pillow...

On their way to the rocket pad, the Doctor sees that the planet is starting to break up around them; if ever he hoped there may be other survivors, that hope is now extinguished. The few people in the bunker are the only ones alive on the entire planet, and Voss only saves the Doctor from the flood because he needed a pilot for the rocketship. When the Doctor gets a good look at the rocket, he realises that it’s been fitted with a nuclear warhead as a weapon of war, and seriously considers refusing to pilot it, if that means taking Voss and Excelsior away to spread their hatred and destructive nature to other worlds. However, that would mean leaving Charley and C’rizz here to die as well, and the Doctor can’t bring himself to do that. The Doctor thus sets about checking the status of the engines and the flight deck, while Voss clears out excess baggage in order to make more room. When he’s cleared out all non-essential material, there’s only room for three or four people, but the Doctor refuses to leave anyone behind and orders Voss to start throwing out even essential materials. As Voss sets off to do so, however, the Doctor hears Charley speak to him, and he drops his work and rushes back to the bunker, realising that something terribly wrong...

C’rizz tries desperately to wake Charley, but only succeeds in drawing forth Excelsior, who is irritated that his cries of distress have interrupted her work on her victory speech. C’rizz accuses her of murdering Charley, but Excelsior in turn suggests that perhaps C’rizz himself chose to put his friend out of her misery. The Doctor arrives, far too late, but though shaken by Charley’s death he tells C’rizz not to jump to conclusions about Excelsior’s part in it. The grieving C’rizz refuses to abandon Charley on this planet, and tells the Doctor that Requiem said the planet may not be doomed; however, the Doctor refuses to take the word of a ghost, and insists that C’rizz honour Charley’s friendship by living on after her rather than giving up. C’rizz reluctantly bids farewell to his dead friend and accompanies the Doctor and Excelsior outside.

As the Doctor drives to the launch pad, Excelsior stands in the troop carrier, waving and delivering her victory speech to a crowd that only she can see. The ground gives way beneath the vehicle, and the Doctor, C’rizz and Excelsior must complete their journey on foot -- but when they arrive, they find Landscar waiting for them near Voss’ dead body. Believing himself to be alone on the rocket, Voss was startled by Landscar’s sudden arrival, stumbled back and fell to his death from the gantry. Landscar informs the Doctor that no one else survived the flood; the Doctor, C’rizz and Excelsior are now the last living beings on Bortresoye. He is disappointed to learn that Excelsior intends to escape; it seems that she has understood nothing of what he was trying to teach her about her connection to this planet. Despite Landscar’s apparent fatalism, however, the Doctor refuses to give up, and he ushers Excelsior into the rocket, determined to save what few lives he can.

Some part of C’rizz understands what Landscar has been telling them about the cyclical nature of death. The Doctor comforts C’rizz as best he can, telling him that accepting the inevitability of loss means that one can appreciate people all the more while they’re still alive, knowing that there will be no second chance to make things right. As the Doctor returns to the rocket, however, Requiem speaks to C’rizz, claiming that death isn’t so bad and will reunite C’rizz with his loved ones. As C’rizz ponders his decision, the Doctor waits for him back at the rocket, ignoring Excelsior’s demands that he take off immediately; he refuses to leave without C’rizz. However, Excelsior claims that her escape was foretold, and quotes the proverb that the end of one journey is the beginning of another -- and the Doctor realises that this is what the Kro’ka was trying to tell him earlier. C’rizz then makes his choice and climbs aboard, and as soon as everyone is strapped in, the Doctor launches the rocket. On the planet’s surface, Requiem and Landscar watch as the rocket departs -- but Landscar assures Requiem that the Last will soon return...

The rocket reaches the upper atmosphere, where the Doctor put it into a geostationary orbit and prepares to use the nuclear warhead to launch them further out into space. Excelsior eagerly anticipates her arrival on another world in need of leadership, and when C’rizz angrily points out that her “leadership” destroyed her own world, she pulls a gun on him, refusing to tolerate his insubordination any longer. The Doctor tries to hold back the furious C’rizz, but Excelsior provokes him by mocking his affection for Charley and admitting that she did in fact kill her -- and she then shoots C’rizz, even though he was only threatening to attack her, not actually doing so. C’rizz dies claiming that he can see Charley looking down on him. For the first time in his life, the Doctor finds that he has it within him to hate, and he refuses to pilot Excelsior any further; they will stay in orbit until the planet dies beneath them. Excelsior tries to threaten, argue and reason with him, but to no avail. However, the rocket’s engines then fire up by themselves, and the Doctor and Excelsior find themselves returning to the planet, whether they want to or not...

The G-forces cause the Doctor and Excelsior to lose consciousness, but the Doctor recovers to find that the rocket has somehow landed safely. Landscar is aboard, waiting for him to wake up, and he explains that the planet itself brought them back. Since the people failed in their duty to protect the planet, it has rid itself of them in order to start over again; however, the cycle cannot resume until the last is dead. Excelsior wakes, groggy and disoriented, and now she really does seem to believe that the people have gathered for her victory parade. She walks out of the rocket, oblivious to the fact that the planet is tearing itself apart around her, and before she can finish the speech she’s been practicing for so long, she is consumed by a volcanic eruption.

Landscar explains that he is in fact an avatar of the planet, its eyes and ears amongst the people, sent to ensure the death of the Last so the cycle could begin again. The Doctor now understands that the people of Bortresoye are reborn after death, learning a little more on each pass until one day the planet decides it is safe to break the cycle. However, the Doctor’s arrival has disrupted the cycle -- and it is he, not Excelsior, who is the Last. The Doctor questions him further, and learns that Bortresoye is indeed the name of the planet on which the various zones have been set up; however, while Landscar knows of the existence of the zones, he has never heard of the Divergents. In any case, he considers this a side issue; the planet itself is at stake, and the Doctor must accept that the cycle will begin again when all living things die. The Doctor realises that he’s being asked to commit suicide, something he’d never considered himself capable of -- but now, he has nothing left to live for, and after a moment’s contemplation, he shrugs and detonates the nuclear warhead in the rocket...