The living god Emperor Pepin VI, eternal lord of all creation, has fallen asleep and drowned in the bath. His son Pepin VII thus becomes the new living God, and all those who worshipped the false Pepin VI now stand accused of heresy. The court scribe, Eugene Tacitus, is taken to the dungeons to confess and be executed, but, as is traditional, he is also given the opportunity to recant. He does so, and Guard Captain Sejanus duly notes his recantation, has him sign a recantation form, and gives him a dark blue receipt and a light blue receipt which he can trade in should he lose the dark blue one. Sejanus also arrests the Empress Berengaria; since her husband was a false god she is a false goddess, and she is to be taken to the dungeons to await execution. Her handmaiden Livilla, Pepin VII's wife, will soon be the new Empress -- and she's going to enjoy dreaming up an inventive death for her predecessor. As Berengaria is marched off to the dungeons, she encounters her other son, the hunchback Childeric, but she doesn't really want to speak to him; traditionally, the illegitimate half-brother of the rightful heir is supposed to be evil, but Childeric has never been villainous enough to satisfy her. But she doesn't know what Childeric has planned for his brother. Her evil will die with her soon enough, but his evil will last for eternity...

There's something wrong with the TARDIS, but when the Doctor tries to warn Frobisher, he finds the Whifferdill in the bath, hunting a gumblejack he's generated from the TARDIS data banks in order to give himself "live" prey. That's just wrong on too many levels. First of all, Frobisher isn't a real penguin, but an alien mesomorph who just happens to prefer the shape of a penguin. Secondly, the cruelty of the hunt is wrong, even if the fish isn't, technically, real. And worst of all, in order to create the fish, Frobisher has interfered with the basic dimensional protocols which hold the TARDIS together... and the ship has had enough. Before the Doctor can correct the damage, the TARDIS shuts herself down in protest. She's tired of being taken for granted, and the Doctor has no choice but to give in and release full autonomous control to his ship, trusting that she won't reduce her interior dimensions or eject him and Frobisher into space. The TARDIS sets off on another journey... but to where, the Doctor and Frobisher will have to wait and see.

Pepin VII is about to become God of all Creation, and he's so nervous he thinks he's going to throw up. At the moment of coronation the crown will burn away his mortal half, but for now he's more scared than sacred. As Tacitus starts work on the first chapters of the new Bible, high priest Clovis shows up to make sure that Pepin is ready for the ceremony. He claims that Pepin's father was just as nervous before his own coronation, but Pepin can't believe that; his father was a god to him. Until he died. Pepin vows to be a benevolent deity, and tries to think noble thoughts. Clovis leaves him, despairing for the state of religion... and finds Childeric lurking outside. Childeric knows that of all people, the high priest must know that the fool he's left behind is no god... they must look elsewhere for their deities. Nervous, Clovis departs, but Childeric knows that he's gotten to him.

At last, the hour of the coronation is at hand, and as Pepin and Livilla approach the throne, Livilla hisses at Pepin to get it right; she's spent two humiliating years as Berengaria's handmaiden in order to be empress, and she's not going to let her idiot husband spoil it for her now. The ceremony proceeds according to form, apart from a brief moment when it looks as though the crown is going to fall off Pepin's head. Pepin's apotheosis must be accompanied by a miracle, and Clovis thus presents a perfectly ordinary deck of playing cards and miraculously identifies the one which Pepin had chosen at random. And thus a new god is born, all-seeing, all-knowing and all-powerful... at least in theory. But in fact, as Pepin addresses his people, promising that his word will be law and his authority absolute, he gives up halfway through and confesses that he feels no different. He was really expecting to become a god, but he's still just as mortal as any of them. Pepin is condemned as a heretic out of his own mouth, and Childeric thus seizes the opportunity to claim that the divinity has passed on to him; he is the legitimate heir, not Pepin. Pepin must prove his divinity or pay with his life... and right on cue, the TARDIS materialises in the throne room.

The TARDIS scanner opens upon a hissing white void, but after a moment the picture clears to reveal the throne room of a medieval castle. Frobisher is worried that the TARDIS might take off without them if they leave, but the doors open by themselves, leaving them with little choice. Frobisher is the first out of the doors... and when Pepin's assembled subjects see a big talking bird walk out of the new blue temple, they fall to their knees and worship him as a messenger from Heaven, proof of Pepin's divinity. Pepin still claims that he isn't a god, but Tacitus arrives, breathless, to inform the people that the new god always suffers from mental confusion after his coronation. Pepin is confused; could he have become God without noticing? The livid Livilla storms off, leaving the Doctor and Frobisher to help the exhausted Pepin back to his chambers to rest.

After checking to ensure that Sejanus is ready for the next stage of the ritual, Clovis leaves him to clear the throne room of riff-raff and follows Childeric to the crypts. There, the bastard hunchback is ensuring that his tongueless servant Arnulf has completed his tasks, just as Arnulf's father used to do before him. It's right that a son should replace his father... Clovis arrives, and as Childeric had expected, he's not here to condemn Childeric for heresy, but to help him. Together they will destroy the new god... and his angels from the blue box.

As the echoes of the gunfire die away, Pepin stands before them unharmed and divinely forgives Sejanus for this attempt on his life. The gun was armed with blanks, since it's impossible to harm a god anyway and there's thus no need to waste live ammunition. The Doctor explains to the shaken Frobisher that since this was only a ritual, there was no danger... or so he thinks until Frobisher opens the door to find that Sejanus stabbed both of Pepin's guards to death to get past them. Tacitus is taken aback by the Doctor's angry reaction; after all, the whole point of the ceremony is to prove that Pepin cannot die, and if the guards didn't die either that would prove they were immortal too, which would just be silly. The Doctor demands that Tacitus show him the ancient texts in which this is all explained, and leaves Frobisher with Pepin... who still doesn't feel divine at all.

In the crypt, Childeric and Arnulf find it amusing when Clovis, disturbed by Childeric's plans, claims to have scruples; after all, Clovis' family is notorious for their treachery. Every high priest betrays his god, and is executed for the crime; then the son follows the father, and history repeats itself. But this time things will be different. Mortal hands cannot harm a living god -- but what might another god do?

Frobisher sympathises with the henpecked and confused Pepin, who doesn't know how to deal with what's expected of him. Livilla is of no help; she only married him for the power and status, and she's so angry about the stunt he pulled during the ceremony that she's considering joining Clovis and Childeric. Frobisher is surprised to learn that there's already a rebellion, but Pepin tells him that it's traditional. The bastard half-brother and the high priest always plot against the heir; they will be defeated and executed, and Clovis' son will succeed him as the new high priest. All Frobisher understands is that their lives are in danger, and he suggests taking shelter in the TARDIS, which the people now worship as a temple. The awed Pepin concludes from the TARDIS' interior that the Doctor and Frobisher truly are angels, but Frobisher insists that they are just travellers. When he casually claims that there's no such place as Heaven, Pepin is stunned. If there's no Heaven, what's the point of living at all?

The Doctor is rather startled to find that the court "library" is Tacitus' bedroom. Nobody at court is interested in the history of false gods, and Tacitus has thus put the sum of all human religious knowledge to work as paperweights and doorstops. The texts describe the minutiae of each god's life in painstakingly thorough detail, but as the Doctor sorts through them he notes something odd. Tacitus has lost count of how many books he personally has written, but even he thinks it's strange that all of the books have the same handwriting in them. The Doctor is more concerned by the fact that each god's life fills their entire bible from start to finish, with no wasted or torn pages at the end... and for some reason, Tacitus has just picked up a thin pamphlet in which to write the Bible of Pepin VII. He claims it was the first to hand, but the Doctor fears there's something more significant in his choice... but before the Doctor can pursue this question further, Clovis arrives and orders them both to accompany him.

Livilla visits the dungeons to speak with Berengaria, who has been expecting her daughter-in-law to begin torturing her. But Livilla is here for another reason entirely; she wants the former queen to claim that she made a mistake at birth, and that Chidleric, not Pepin, is the rightful heir. Berengaria is disappointed; when she became goddess, she was inventively slow in disposing of her predecessor, and she's actually disgusted when Livilla pleads with her and offers to spare her life. Berengaria tired of being all-powerful many years ago, and she just wants to get it over with; furthermore, the soldier who fathered Childeric was the only man she truly loved, and she will not dishonour his memory by claiming that Childeric was anyone else's son. Livilla doesn't have what it takes to be Empress, and Berengaria orders Livilla to go, unless she's willing to hurt her properly. Livilla, furious, does both.

Clovis brings the Doctor and Tacitus before Childeric, who isn't interested in the Doctor but needs Tacitus for the next stage of his plan. Their religion has become empty over the years; nobody really believes in God any more, and the rituals have become hollow and meaningless. As the people began to doubt, so did the gods begin to doubt in themselves, and where high priests used to burn with passion now Clovis is frightened of the dark. Childeric intends to become god... but a real god, not the powerless figurehead of the state religion. He thus takes his prisoners to the deepest levels of the dungeons, where nobody has ventured for centuries; this is beyond anything in Tacitus' texts, and he's terrified of what he might find here. And even here, Childeric opens up a secret panel, which leads into even lower levels, going deeper still into the dark heart of the castle. Here is where they will meet Childeric's son, hidden alone since birth, untainted and pure... their new messiah.

Frobisher has come to believe that the TARDIS wanted attention, and that it brought them here so it could enjoy being worshipped as a temple. But Pepin is more concerned with his people, who will soon expect him to start making proclamations and visiting his divine wrath upon them. There's no point in a god who isn't vengeful, and Pepin will soon have to execute one tenth of the population for heresy; after all, last week they worshipped his father, and his father was no god. The people are destroying all images of the former god emperor wherever they can be found, and Pepin knows they will already have broken into the morgue and torn apart his corpse, angry that he did not rule them forever as they thought he would. Now they all worship Pepin, and will build statues in his honour... but Pepin knows that one day, he too will fail them. He still thinks of his father as a god, and knows that he's unworthy to follow in his footsteps; all he can do is admit the truth to his people, abdicate, and let them kill him. Frobisher begs him to reconsider, but Pepin is adamant and steps out of the TARDIS to address his people. He is no god, but he knows that they need one. He therefore steps aside for a new god, a wise and courageous being who came from another world to bring salvation to them all...

Childeric's wife mysteriously disappeared five years ago, or so everybody believed. In fact, she gave birth to a son, and he cut the tongue from her mouth as she did so, so that even in the agony of childbirth her curses could not corrupt the child. Childeric allowed her to feed his son until she ran dry, and then walled her up alive in the dungeon and sent servants such as Arnulf to feed the child. They too had their tongues removed, so as not to taint the new messiah with the cynical, complacent speech of Man. Isolated from the corruption of the world, the child will be perfect and pure, a real god who will teach Childeric how to be divine. But events have overtaken Childeric, whose father died before his son came of age. Now he intends to cut out Tacitus' tongue and seal up the scribe with his child, to chronicle the life of the new god. The Doctor will just have to die.

Frobisher tries to flee back to the TARDIS, but it's locked him out. Sejanus lays down his sword before his new god, awaiting his orders; naturally, his first task will be to take Pepin away and execute him for heresy. But to everyone's shock, Frobisher forbids this. It goes against everything in the constitution, but Frobisher is starting to get the hang of his new status, and he not only pardons Pepin but appoints him the new high priest. After all, nobody believes in Frobisher more than Pepin does; and as long as he's going to be God, there are going to be a few changes around here. Meanwhile, Livilla approaches Childeric, still seeking power, but he has little interest in her; years of the bitterness he was born to have robbed him of all interest in beauty, and he only takes mild amusement at her offer to let him make her as ugly as he is. However, she gets his attention when she informs him that Pepin has abdicated, and that another god now rules in his place...

The Doctor and Tacitus have been chained up in the dungeons. Tacitus is rather philosophical about his fate, and regrets only that he won't see how it all turns out in the end; his son will have to carry on his work. The Doctor, interested, presses Tacitus for more information about his son, but the scribe grows ever more uncomfortable, and finally explodes violently, refusing to discuss his son any further. They're just a family of scribes, petty and worthless and of absolutely no value whatsoever. To his sorrow, the Doctor thinks he's starting to understand. Clovis arrives to cut out Tacitus' tongue and torture the Doctor to death, but admits that he doesn't really want to; however, his family has always betrayed their gods, and even if this time the rebellion is somewhat more confusing than usual, at least Clovis knows his role in things. Before he can set about his work, however, Childeric shows up with new information. Frobisher has usurped the throne, and whereas Childeric could always have dealt with Pepin, the big talking bird is an unknown quantity. The time for caution has past; Childeric is going to release his son, the god.

Frobisher tries to introduce his people to the concept of parliamentary democracy, but their response is to ask him who he wants them to elect. Despairing, Frobisher inquires after the Doctor, whom he suspects may have left the castle -- but that's a concept which Pepin simply can't comprehend. Before Frobisher can pursue this question further, Sejanus bursts in for the traditional assassination attempt... but this time, because of the Doctor's earlier complaint, he's using live ammunition. Before Frobisher can react Sejanus shoots him -- and the bullets pass straight through him without harm, proof positive that he is indeed a god. Seriously disturbed, Frobisher sends Pepin to pay off Sejanus while he tries to come to terms with his miraculous survival...

Livilla accompanies Childeric and the others to the crypts, but she's confused and angry, and doesn't understand what any of this has to do with usurping the throne. Arnulf releases the new messiah from his crypt... and Tacitus reacts in sheer horror to the sight of the child's face. He's seen that same face before, too many times, and he warns Childeric that waking the child will mean the death of them all. But Childeric does so -- and is shocked when, despite all his attempts to shield the Child from the corruption of the world, the Child speaks to him in the language of Man. But of course it would. Gods know everything. And mortals are merely their playthings. When Livilla protests, demanding to be made goddess as is her right, the Child shows her what real power is by transforming her into the squalling infant she has always been. It breaks its new plaything, however, and demands another -- and when its father refuses it, the Child flies into a tantrum which threatens to bring the walls down. Tacitus flees in terror, the Doctor and Clovis right behind him... but Tacitus will not tell the Doctor what he knows, only that they have to get away before they all die. For the Child will keep coming, even though he's killed it already, more times than he can count. The Doctor now knows what the Child really is, and he goes back to speak to it -- even though he knows that if he's right, he hasn't got a chance of getting it to listen to reason.

Frobisher's subjects are already making statues in his honour, but the first one doesn't have the beak quite right and the sculptor is therefore condemned to death. However, Frobisher morphs his beak into a different shape so that it matches the statue exactly, thus saving the sculptor's life. He will not let anybody die because of him, and when he learns that someone is already in the dungeon awaiting execution, he insists that she be released at once. The prisoner is Berengaria, and Livilla's earlier visit has left her weak and dying from loss of blood. Pepin begs his god to save his mother's life, and Frobisher reluctantly lays on his flippers... and is shocked when Berengaria's wounds vanish. Perhaps he really is a god.

Childeric is unprepared for the Child's tantrums, and the actuality of its omnipotence. All the Child wants to do is kill, and Childeric is helpless to stop it when it restores Arnulf's power of speech just so it can listen to his screams as it kills him. Then the Doctor arrives, and demands to know what the Child intends to do with its powers. It's quite simple; the Child intends to destroy everything that there is, the population of the castle and then the castle itself, until it is alone with its father. Just as the Doctor had suspected, neither Childeric nor the Child have any conception of a world outside the castle; the Child may be a god, but it's the god of a very small domain. The Doctor now knows what the castle really is, and why the TARDIS brought him here... and that while none of its inhabitants can harm him, the Child is something very different, and very dangerous.

The Doctor now knows what the Child really is -- not a god, but a device constructed to torture a single man. He refuses to tell the Child who its father is, but the Child insists that it doesn't want to kill its father, but to beg for his forgiveness. It must have been a very bad Child indeed for its father to have killed it. And until it has found its father it will kill everyone in the castle, to make its daddy proud. The Child forces its way into the Doctor's mind to read his thoughts, but is stunned by what it finds there -- a universe full of planets, stars, galaxies, and millions upon millions of people. This cannot be; the castle is all the Universe, and the Child's father is the centre of it. Enraged, the Child vanishes into thin air, leaving the Doctor with two miles of winding stairs to climb, and the desperate hope that he will be able to find Frobisher before it's too late.

Berengaria is confused and angered to find that she's been saved; her role as empress has come to an end, and it's time for her to die. By overturning the rituals, Frobisher has betrayed tradition and left them all mired in uncertainty. Of course the old rituals made no sense, but at least everyone knew their place in them. Berengaria wraps her broken chains about herself, preferring to waste away and die as she's supposed to, and demands that Frobisher leave her alone with her son. Confused, Frobisher does so. Pepin tries to care for his mother, but she rebuffs his every attempt to be kind, as is traditional. Why should she love the thing that stretched out her womb for nine months and ruined her figure forever? She never loved Pepin's father, a stammering idiot frightened into failure by fear of his own father's disapproval. It was Pepin's birth that made him the monstrous tyrant he became. Pepin turned his father into a god, as his father had done to his own father before him. It's what fathers and sons have done to each other over the centuries. It's traditional.

As the Doctor climbs, he encounters Clovis, who begs him to help destroy the Child -- but the Doctor knows that Clovis will betray him at the first chance he gets. He has little choice in the matter; the man who created him had little imagination and a taste for the melodramatic. Clovis, who is desperate to be a good man, demands that the Doctor explain himself, and the Doctor reluctantly reveals that this castle and everyone within it is a fiction. Clovis is a stereotype, his actions predetermined by his nature. As genuinely sorry as the Doctor may be for Clovis, and as much as Clovis may want to behave properly, the Doctor simply can't trust him. Clovis allows the Doctor to go his own way, wishing him good luck, and braces himself to defy his nature and die nobly. But when the Child materialises before him, demanding to know where its father is, Clovis collapses in terror, pleads for his life, and points the Child in the Doctor's direction. He does not die nobly.

Frobisher retreats to the throne room to think about what's happened, and is surprised when Tacitus runs in and hides behind the throne. Tacitus is gibbering in terror, insisting that he's nobody important, just a menial scribe, and certainly not worth all this carnage. The Child is coming for him now, killing everyone in its path, and it won't stop until Tacitus kills it... again. Outside the throne room, the Child is flitting about the castle, killing at random; the walls and floors of the castle run with blood, and the terrified people cling to statues of Frobisher in the hope that their god will save them. But Frobisher is no god, and when Sejanus realises this, he storms out of the throne room in a rage; he knows he's going to his death, but he'd sooner die defending his people than the false god who promised to protect them and did nothing. Tacitus barricades the door, weak with self-reproach. It's natural to love one's own son; why couldn't he?

In the crypts, Tacitus' voice seems to echo in Berengaria's head. It's natural to love one's own son; why couldn't she? For the first time ever, she starts to understand just what it is that Pepin wants from her, and thinks that maybe, just maybe, he'll be able to teach her how to love him. But then the Child appears, bright and shining and dripping with blood. Pepin stands up to it, refusing to let it harm his mother, and although it destroys him with a thought he dies without screaming. The Child orders Berengaria to bow down before it, but she refuses; she will not live in a world where a monster like this is god. Enraged, it kills her, much to her relief.

The Doctor arrives at the throne room, to find that only Frobisher and Tacitus are there. They are the only three people left alive in the castle... or rather, they would be, if anyone else in the castle had ever been alive. The TARDIS brought them to this castle because it is also dimensionally transcendental, and the TARDIS needed a similar environment in which to recover from the damage which Frobisher had inflicted upon it. The Doctor explains to the stunned Frobisher that this entire environment is a fiction, an elaborate prison constructed by and for Tacitus. He's been trapped here for centuries, living in this elaborate make-believe world for so long that he'd forgotten he wasn't a part of it. Here, he creates worlds in which to lose himself in meaningless obsessive rituals and forget what he had done -- but he cannot bury the guilt forever, and when it becomes too much, the fantasy breaks down, and the Child returns, forcing him to relive his crime. The Child has now destroyed the entire castle, and with nothing left to distract him Tacitus finally remembers who he is... and what he did.