The story begins with the sound of trumpets in a misty valley, where an elderly adventurer named the Doctor is waiting for an army to arrive. But as the army emerges from the mist, it is ambushed by Gaulish warriors, and the Doctor finds himself caught in the midst of battle. One of the soldiers strikes down the Gaul with an axe, saving the Doctor’s life, but the soldier and his talking elephant Surus are suspicious of this elderly man who seems to know too much about the army’s plans. The Doctor realises that his saviour is the man he’s come to see -- the Carthaginian general Hannibal Barca, who is preparing an unprecedented attack on the city of Rome.
Hannibal allows the Doctor to share Surus’ back as the army resumes its march, and even agrees to transport the Doctor’s strange cabinet with them. However, the Doctor then faces endless questions from the suspicious general, who demands to know where the Doctor came from and how much he knows about Hannibal’s plans. The Doctor tells Hannibal all he knows about the rivalry between Rome and Carthage, and of Hannibal’s intent to lead his army over the Alps and attack Rome from the north while they are preparing to attack Carthage in the south. Hannibal demands to know how much the Romans know and what their plans are, but the Doctor refuses to interfere. He is here simply to observe history as it unfolds and acquaint himself with the details. But Hannibal can sense that the Doctor knows his future, and warns him that if he doesn’t reveal it, then Hannibal and Surus will consult the Doctor’s entrails themselves.
At this point, the creative flow of the story is interrupted by the Doctor’s manservant Badger, who has prepared supper for him. The Doctor is infuriated, particularly when Badger corrects him about the colour of the Gauls’ hair. He thus takes a break from writing and turns to the latest news, disposing of the political bulletins and moving straight to the Universe at large. He is disturbed to see that two more star systems have fallen to the Thalek Empire and that the Time Lords have done nothing about it; if only he was younger... But as he searches through his papers he stumbles across something even more personally disturbing. It seems that he’s been invited to a Presidential reception to mark the 300th anniversary of the publication of his first science-fiction historical romance, An Adventurer in Time and Space, but that Badger has turned down the invitation on his behalf. Badger claims that he was acting on the Doctor’s usual instructions... and his voice drops into a hypnotic register until even the Doctor seems wavering and unsure. But when Badger tries to prompt the Doctor into admitting his interest in politics, the Doctor snaps out of his trance; he has no interest in politics whatsoever, and it’s time to get back to the story.
The army makes camp for the night. Hannibal offers the Doctor wine, but the Doctor turns down the offer, aware that the general is trying to loosen his tongue. Frustrated, Hannibal demands to know what the Romans are planning, but before he can extract any useful information from the Doctor, another band of Gauls attacks the camp. Hannibal leaves Surus to guard the Doctor, and though Surus is wary of the stranger, he is proud to speak of his warrior heritage. He tells the curious Doctor that Hannibal has hated the Romans ever since his father Hamilcar taught him to do so, and the Doctor is somewhat disappointed to learn that this whole war is, at heart, down to family tradition. But as he speaks with Surus he hears a mysterious female voice echoing in his head, calling him “Grandfather”...
The Doctor is pulled sharply out of his story to find that his great-grand-uncle, Ordinal-General Quences, has dropped by for a visit. Quences inquires after the Doctor’s work, and professes irritation when he learns that the Doctor’s latest novel takes place on the backwater Earth. The Doctor has been a great disappointment, frittering away his expensive education and never fulfilling the potential the family saw in him. It’s an old argument, and the Doctor is tired of explaining that he has no interest in the family’s political ambitions; come to that, it occurs to him that he doesn’t even know what an Ordinal-General actually does. Fed up, the Doctor offers to show Quences what he’s always wanted, and Quences is stunned to find himself and the Doctor suddenly standing in the Panopticon itself on the occasion of the Doctor’s investiture as President of the High Council of Time Lords. Quences pulls himself together and fusses over the Doctor’s descent to the podium, ordering him not to embarrass the family and reminding him of all the other Presidents that have ruled and died before him. This point seems quite important to Quences -- traditional, in fact. Quences wears robes of darkest night and bears the promise of death in his pocket; he is the living reminder that even the greatest power is transient and that all must decay in the end...
The Doctor, unnerved by the dark change in Quences’ behaviour, switches off what he now reveals to have been a mere possibility projection -- but as he berates his uncle’s ambition, he once again hears the voice in his head, and there comes a knocking on the door. The Doctor orders Badger to get rid of the unwanted visitor and resumes bickering with Quences as Badger shambles off to the door. However, the woman at the door won’t take no for an answer, and when Badger tries to slam the door in her face, she shoots him. She’s here to see her grandfather...
Susan pushes her way past the disabled Badger to find herself in a room filled with stacks of paper, research notes piled so thick that there’s barely room to move. Badger recovers from the blast and attacks her, and she finds that her gun no longer works. As she retreats she knocks over a stack of notes to reveal the Doctor, who sits unmoving, strapped into some sort of machine. Susan touches the Doctor and is swept into the possibility projection with him, and the furious Quences materialises and orders Badger to follow her in and prevent her from interfering. Susan finds herself trapped on a mountainside in the middle of a blizzard, and as she tries to find help she falls and twists her ankle. Badger tracks her down, but before he can attack, Hannibal’s army arrives.
Story and reality have overlapped, but as yet the Doctor is unaware of this. Hannibal’s army has reached a crossroads, and as the Doctor seems to have knowledge of the future, Hannibal demands that he tell them which route to take to Rome. The frustrated Doctor informs Hannibal that he’s a fictional creation, an alternate possibility created in a machine so the Doctor could research a novel without leaving his home. This is why Surus the elephant has the power of speech; the Doctor needed a character to act as Hannibal’s confidante. But as Hannibal’s frustration grows, the Doctor begins to realise that the nature of this fiction doesn’t absolve the Doctor from responsibility -- quite the opposite, in fact. In a sense, he created all of Hannibal’s army, and now they’re suffering and dying because he didn’t complete his research. He’s left the future up to Hannibal to decide, but what happens to that future if Hannibal makes the wrong choice?
Before the Doctor can decide what to do, Surus returns from a recce, dragging Susan and Badger with him. Susan recognises the Doctor and calls him grandfather, but the bewildered Doctor has no idea who she is. He promises to tell Hannibal which route to take if Hannibal gives him five minutes to talk to the strange woman, and Hannibal agrees to let them speak in his tent. The Doctor demands to know who Susan is, and she reminds him of the family that he left behind -- and his grand-daughter, the only one in the whole dark clan who saw the world with the same wonder as he. The Doctor finally remembers Susan, and they share a joyous reunion before the Doctor pulls himself together, realising that he must now get Susan out of the mess he created for himself. He is now able to provide Hannibal with the answer he wants; Badger retains all of the Doctor’s research in memory, and he advises Hannibal to take the pass at Montgenevre. As the blizzard clears, Hannibal sends Surus to muster the troops and warns the Doctor that there are five alternate routes to take -- and that he’ll keep restarting the programme if he has to until he finds a possibility that works.
The Doctor, Susan and Badger return to reality, and the Doctor sends Badger to fetch some food for Susan, noting that Badger’s attention is wandering -- but unaware that it’s because he is communicating with Quences. Susan is relieved to see that the Doctor is still alive after all, but when he asks Susan why she’s come to visit, she changes the subject and asks about the machine he was strapped into. He admits that he’s been using a possibility generator to create alternate realities in order to visit the historical characters he writes about, getting the details and nitty-gritty of their existence rather than having to rely on cold, second-hand reports from the Time Lord observatories. He modestly dismisses his work, but Susan assures him that his stories are loved across the planet; even her own grandchildren enjoy them. The Doctor is delighted to learn that he’s a great-great-grandfather, and finally realises just how much he’s been missing by shutting himself away over the years. He admits that he once planned to steal a TARDIS, flee from Gallifrey and take young Susan with him; how different would all their lives have been then? But even in the midst of his joy, he senses that Susan is hiding something from him...
Meanwhile, Quences is communicating with Badger, demanding to know Susan’s agenda. The public registry records confirm that she is who she says she is -- which means that she must have an ulterior motive for her actions, just like everyone else in the family. If she threatens to interfere with Quences’ plans for the Doctor, she will have to be killed. But Badger then finds something particularly interesting in the public registry, something with astounds and infuriates Quences. He’s spent so long trying to mould the Doctor that he failed to notice another candidate in the world outside. The Doctor isn’t Quences’ ticket to high office after all -- it’s Susan.
Susan can’t bring herself to tell her grandfather the truth, but he won’t let her leave just when they were getting reacquainted, and he decides to accompany her. However, as she starts to lead him out of his rooms for the first time in centuries, Badger arrives and insists that she remain for supper. The Doctor becomes confused and distracted, and Susan realises that Badger has some kind of hypnotic influence over him. As she tries to drag him away, Quences materialises before her, taking charge personally -- much to Susan’s shock, since Quences died long ago, when she was still a child. The Doctor is shocked out of his reverie when he learns that Quences is long-dead, and that rather than transferring his incorporeal mind into the Matrix, he illegally transferred himself into Badger -- the positronic servant he gave to the Doctor as a “gift” -- and has been plaguing the Doctor ever since. And as it turns out, all of Quences’ effort was for nothing, since it’s Susan who has achieved what he hoped the Doctor would. Within hours, she is due to be sworn in as President of the Supreme Council.
Quences intends to take full advantage of Susan’s ascension to high office, but the Doctor acts first, activating the possibility generator and fleeing with Susan into another possibility. They end up on the other side of the Alps, and since the Doctor has taken the controls into the possibility field, there’s only one way out. While waiting for Hannibal’s army to arrive, the Doctor questions Susan about her investiture, and she reveals that she’s written to him many times, only to receive brusque replies which she now realises were sent by Badger. The Doctor fears that the family manipulated her into seeking political office, but she insists that she has her own reasons for doing so; she’s learned well from the Doctor, and she intends to bring life and energy back to the stale, stagnant Gallifrey. The Doctor, touched and impressed, vows to take her back home to fulfil her destiny -- without interference from the family.
Hannibal’s army arrives, but it’s in poor shape; they lost 12,000 men and half their elephants crossing the Alps, and Hannibal no longer cares for the Doctor’s version of history. And now he has another guide -- Quences, who sent Badger into the possibility field after sending a message to the Prydonian chapterhouse in the Doctor’s name, assuring them that Susan would be present for her investiture. Susan doesn’t trust his intentions, and realises that she’s seen him before, or someone like him, in old archive records.
The Doctor reclaims his travelling cabinet from Hannibal, and Susan is delighted to learn that the Doctor’s fictional counterpart has a TARDIS, the only means of escape from this cold world. The Doctor reluctantly agrees to take Badger and Quences with him, and as he unlocks the TARDIS he advises Hannibal to watch out for the Romans at the Ticinus and Trebia rivers. But when he opens his magical cabinet he finds only an empty interior. The cabinet was never real, just a literary conceit to enable the fictional adventurer to travel from one place to another. All seems lost, but the Doctor refuses to surrender to despair; while there are still alternate possibilities, there is still hope, and this reality is ruled by the way he looks at it. The Doctor takes Susan’s hand and invites her to imagine what would have happened if he’d made some different decision in the past -- eaten his supper while it was still hot, or stayed an extra hour in bed. Then they wouldn’t be here at all; they’d be somewhere else entirely. And before Quences can stop them, they are. With imagination, who needs a TARDIS?
Susan is astonished to find herself in the Doctor’s favourite retreat, a rose garden scented with time’s memories. They’ve escaped from Quences and Badger, but the Doctor has wearied of jumping between fictions, and can’t see a way out of the morass of possibilities in which he has become mired. He can barely tell the difference between reality and dreams any more, and he suspects that Susan is only here because he wished it. In the centre of the time garden is a tree of possibilities, where each decision branches off into an infinite number of universes. The Doctor has seen many marvels on the possibility tree, but in every universe the people of Gallifrey sit back and do nothing while the Universe burns around them. But in one possibility he saw himself travelling the Universe in an old TARDIS, just as he’d once planned to do. The Doctor nearly loses himself amongst the myriad of possibilities, but Susan brings him back to himself; she has remembered who Quences really is, and she guides the Doctor backwards along the branches of possibility, back the way they came, back home.
The Doctor and Susan find themselves back in the Doctor’s quarters, but Badger and Quences are there too; Susan was thinking about them as she returned, and this gave them the possibility of escape. Susan now knows who Quences is; she’s seen pictures of him and people like him at the investiture of every President ever elected to the Supreme Council. He is Auld Mortality, the embodiment of death, a living reminder that all power is fleeting and that everything must come to an end. And now that Auld Mortality has a family member in the Presidency, power and death have become allied, and Quences will have supreme power.
Trumpets sound, heralding Susan’s investiture, and Quences accompanies her to the Panopticon -- but as he lurks behind her, whispering dire warnings and reminders of death in her ears, the Doctor is hovering behind Quences, distracting him and cutting him down with flippant remarks. Susan takes the stand and vows to watch over the sanctity of Time until death takes her, and then turns on Auld Mortality, attempting to use the power of the Presidency to banish him. The Doctor takes the dais to warn the Time Lords of the cancer at the heart of the Council, and the furious Quences orders Badger to kill him. But Badger is torn between conflicting duties, and as he hesitates, fiction and reality overlap and Hannibal and his army storm the Panopticon. Auld Mortality has outstayed his welcome, and Surus obeys Hannibal’s orders and skewers Badger with his tusk. Quences flickers and vanishes like a flame going out, and as Badger dies, the curse of Auld Mortality is lifted.
Back in the Doctor’s quarters, the Doctor thanks Hannibal for his help, which was made possible because of the Doctor’s advice to Hannibal to watch out for Romans at the Ticinus and Trebia rivers. As Hannibal and his army march back into possibility, however, the Doctor admits to Susan that he will lose his war, and that the Romans will raze Carthage to the ground and sow it with salt. The Doctor must switch off the possibility generator now before it does any more damage, but he hesitates to do so, still fearing that Susan was just a possibility; however, once again Susan brings him down to earth by switching off the generator himself. Time begins to move forward once more, and, no longer lost in dreams of alternate possibilities, the Doctor finally realises that he’s been living in the console room of a TARDIS, the one he intended to steal before Quences infiltrated his life through Badger and took that possibility from him.
Susan realises that there’s nothing to stop the Doctor from leaving Gallifrey now, and urges him to take the chance while he still can. He invites her to come with her; the Council can find another victim to become President, and there’s a whole Universe to explore. Perhaps she accepts, perhaps she declines; perhaps they depart together in joy or bid a sad farewell to each other before the Doctor sets off alone. Out in the Universe there are Thalek armies, Winston Churchill, schoolteachers, a junkyard with the name I.M. Foreman on the gates and a foggy night on Barnes Common. The possibilities are endless.