Somebody is writing a story set in the bucolic English village of Arandale, where a serial killer is murdering the townspeople and draining their bodies of blood. The mysterious Matthew Shade has arrived to investigate the murders, and hard-boiled American private eye Jack Corrigan has come to town on a separate case. White witch Rosemary Chabmers senses that some great change is coming to her home, but is unaware that the manic priest Father Sheridan has come to town to kill her in the name of the Lord. The TARDIS materialises outside town, and when the Doctor stops Ace from experimenting with the repaired chameleon circuit and places it on a default setting to blend in with its surroundings, it transforms the TARDIS into the shape of a gingerbread cottage. Irritated, the Doctor restores it to a police box shape and takes Ace and Benny into town, where the police have just discovered another body. The Doctor and his companions check into the local bed and breakfast, where the Doctor plans to investigate the killings while Ace and Benny visit the Black Cat Tavern. Ace befriends Mel Joseph and his fiancée Karen Davies, and becomes curious about Corrigan, who seems too clichéd to be real. Meanwhile, Benny is trapped at the bar with the long-suffering Phil Chambers, whose wife has become a tourist attraction and local laughingstock due to her beliefs.

Ace and Benny make their separate ways back to the boarding house. Ace is nearly run down in the street by the drunken Mason Grimshaw, while Benny meets a suicidal old man named Norman Power who believes that he was once a superhero. He claims to have given up his powers as the White Knight when his plucky ward Sparky was killed by his nemesis, the evil Doctor Nemesis. Benny’s heart goes out to the clearly delusional old man, and as she speaks with him, trying to help him through his depression, she finally confesses that the conflict between her friends is too much to handle and that she’s going to leave them. Power urges her to reconsider what she’s giving up before she makes the same mistake he did -- and when he realises that she doesn’t believe his story, he opens up a hidden panel on his bookshelf to reveal a secret entrance to the underground lair of the White Knight. While Benny is still trying to work out how to react to this, the evil Doctor Nemesis bursts into the cavern, seeking revenge. Benny distracts Nemesis while Power escapes, fleeing through the hole which Nemesis had blasted in the cavern wall.

The next day, Mel and Karen show Ace about the town and then return to the Black Cat for drinks. There, Mel excuses himself to speak privately with Mason Grimshaw -- who beats him to within an inch of his life. Mel, however, refuses to tell either Karen or Ace what went on between them. Meanwhile, the Doctor explores Arandale, buys a Travel Scrabble set and refuses to explain to anyone why he’s carrying a spoon in his pocket. He soon meets Shade, who warns him that there will be one death each night until Guy Fawkes’, at which time the evil Force which lurks beneath the town will be released. The Doctor returns to the bed and breakfast to ponder what he has learned, and Benny meets him there, introduces him to Norman Power and tells him her story. Power claims to have gained his super abilities after the meteorite storm which destroyed Arandale Keep in the 1950s; ever since that night, anybody who has ventured near the Keep has sickened and died. The Doctor suggests that, exaggerated as his exploits may be, there is a grain of truth to them; his genetic structure may have been altered by exposure to a rare and dangerous form of extraterrestrial energy known as McAllister’s Radiation. If the Force beneath the Keep is indeed composed of McAllister’s, the Doctor warns Benny, they’re all in great danger.

Benny offers to let Power stay in her room until Nemesis has been defeated, and asks Ace if she can stay with her. Ace, offended by Benny’s presumption, storms off to the TARDIS to sleep in her room, but finds that the TARDIS is nowhere to be seen. She unexpectedly ventures beyond the limits of the setting while searching for it, and after some frantic rewriting she awakens the next morning with no memory of what she’s seen. Confused and disoriented, she staggers back to town, and in what seems to be a moment of weakness she opens up to Benny and admits that she’s staying with the Doctor to prove that she can out-manipulate him and beat him at his own game. Benny, thinking that she’s doing the right thing, tells the Doctor what Ace had said -- but when Ace learns this she is furious, and a wedge is driven between her and Benny as well, possibly for good.

The Doctor visits Rosemary Chambers to discuss the supernatural element of the murders, and Phil storms out in disgust on this insanity -- clearing the way for Sheridan to enter the shop and attack Rosemary, in the belief that he is doing the Lord’s work by putting a witch to death. The Doctor intervenes, but Sheridan pushes past him and is about to strike Rosemary dead -- when the Doctor points out that Rosemary’s supposed powers are having no effect on Sheridan, even though the Doctor removed his crucifix while they fought. Sheridan realises that he has nearly killed a deluded but innocent woman, and offers no resistance when the police arrive.

The local crime-solving gang known as the Adventure Kids decide to investigate the Mystery of Arandale Keep, certain that the tunnels beneath the ruined castle are occupied by criminals wearing unconvincing ghost costumes to frighten the credulous townsfolk, just like the other gangs the Kids have exposed during past school holidays. But instead, when they enter the tunnels they encounter a hooded and cloaked madman, who is holding a bloody knife and chanting a prayer to his dark gods. The Kids flee in terror. Meanwhile, Norman Power sits alone in the ruins of the Keep, hoping in vain that the Force which once made him a superhero will take notice of him again.

Karen walks out on Mel when he refuses to tell her what’s going on between him and Mason Grimshaw, but on the streets outside she is attacked and left for dead by the serial killer. Meanwhile, Ace realises that Corrigan is following her, and when she confronts him he admits that he’s trying to find Mel Joseph -- the evil head of an illegal international gambling cartel. Since he doesn’t know what Mel looks like, but Ace mentioned his name when they met in the pub, Corrigan has been following her in the hope that she will lead him to Mel. Ace is unable to reconcile the Mel she knows with the one Corrigan describes, and goes to question Mel herself -- but he is indeed a megalomaniac obsessed with games, and he flees through the rooms of his house, which have been rigged with a series of deadly traps and games. As Ace pursues him she realises that the house is bigger on the inside than the outside, and Mel tries to cover by claiming that it’s just like her TARDIS -- which he can’t possibly have known. Corrigan arrives just in time to save her, but although Mel is arrested Ace now knows that what she’s experiencing isn’t real. She returns to the town limits where she fainted earlier, and finds that the countryside is apparently floating through the depths of space. Convinced that this is another illusion, she leaps off the edge of the town into a number of dream sequences, none of which faze her. She demands to be shown the truth, and finds herself in a void where White Robots appear and march her back to Arandale.

The Doctor visits Mel in prison and offers to play a game of Travel Scrabble with him. As they play, however, Mel places a word which can’t possibly fit on the board’s layout, a continuity error which confirms the Doctor’s suspicions that he is once again in the Land of Fiction. He demands to speak to the new Writer, who turns out to be an adolescent transported from 20th-century Earth by the person who raised the Land from the ashes of the Doctor’s last visit -- the same enemy who has been changing other aspects of the Doctor’s past. The Writer has been here for some time now, writing adventure stories and practising his skills against a fictional counterpart of the Doctor named Dr. Who, and he is now ready to complete his work. Although he claims that he can do anything here -- end the Discworld series, or relive Danny Pain’s fight against the aliens in 1976 -- the Doctor realises that all he really wants to do is go back home. To this end, he has trapped the Doctor in his last story, and once it reaches a natural close, the Land of Fiction will transform the Doctor and his companions into fictional characters, to become the Writer’s playthings. The Doctor has only three options: solve the mystery and become a fictional character; fail to solve the mystery and die in the cataclysm soon to befall Arandale; or take the Writer’s place, and become the new Master of the Land for the rest of Time.

The Writer returns the Doctor to Arandale and begins to wind down his sub-plots. Father Sheridan is released on bail, and as he staggers through the streets in a crisis of faith he becomes the serial killer’s final victim. Mel has been blackmailing Mason Grimshaw, who killed a child while driving drunk -- but the stress has driven Grimshaw over the edge, causing him to re-adopt the persona of Doctor Nemesis and set off after his old enemy Norman Power. Rosemary Chambers, sensing that her help will be needed, drags Phil to the bed and breakfast to speak with Ace and Benny, but Nemesis bursts in and demands that they surrender Power to him. Norman attacks Nemesis, trying to save his friends, but Nemesis nearly beats him to death -- until Rosemary uses her own powers to blast Nemesis into a cloud of greasy ash. At that same moment the Force explodes up out of the ruins of Arandale Keep, Sheridan’s death having given it the energy it needs to free itself. The Adventure Kids, who had returned to investigate further, are incinerated in the explosion as the Force heads into town to claim its host.

The Doctor collects Benny and Ace and explains the true situation to them, as there is no longer any need to keep his knowledge a secret from the Writer. Karen wakes with no memory of her attacker, while Norman, in the deluded belief that his powers have returned, leaps from his hospital room window and falls to his death. The Doctor tries to block the Writer by writing a new tea room into existence, where he and his companions can take a breather; however, the Writer’s influence is too strong for him and the room fades from existence. Determined to force the Doctor into a confrontation, the Writer sends the killer to confront him and his companions. As the Doctor had already guessed, the killer is Phil Chambers, who as an unborn child was possessed by the Force during the meteorite storm. This is why Rosemary was only able to cast spells while he was in the vicinity; they were not her powers, but his.

Chambers is completely subsumed into the Force and goes on a rampage through town, killing everyone he encounters. The Doctor and the few survivors -- Corrigan, Shade and Karen -- hide out in a garage where Shade has built a machine capable of dispersing the Force; the spoon which the Doctor has been carrying all along turns out to be the vital final component. The Doctor refuses to let Shade use it, however, and instead takes his friends to the town clock, claiming that he can rewire the chimes to create sonic vibrations which will destroy the Force. Instead, they chime out the theme to “The Magic Roundabout”. Furious, the Writer transports Ace to a fictional court, where she is accused of outlasting her potential as a character and is attacked by flying books entitled Dragonfire, Love and War and Deceit. In order to get her back, the Doctor allows the Writer to set up a final confrontation between him and the Force… but once again, he refuses to destroy it. The infuriated Writer therefore has the Force blast the Doctor and his friends into oblivion…

And when the Writer recovers a few minutes later, the Doctor, Ace and Benny are back in the TARDIS and on their way out of the Land. As the Writer’s powers start to return he tries to rewrite the escaping ship back into his clutches, while the Doctor tries to write it to safety. The Writer gains the upper hand, drawing the TARDIS back towards the Land in the grasp of a giant pair of pincers -- but as the Doctor collapses, helpless, Ace uses the chameleon circuit to transform the TARDIS into a smooth ball bearing. Before the Writer realises what is happening, the pincers clamp shut, snapping the TARDIS out of the Land of Fiction forever.

Disgusted with the Doctor for leaving it up to her to save them, Ace storms off to her room. The chastened Doctor admits to Benny that he’d guessed what was happening when the TARDIS transformed into a gingerbread cottage, and he therefore changed the story by specifying the nature of the Force as McAllister’s Radiation. When he goaded the Writer into releasing a huge blast of the energy, it cancelled out the influence of the Land of Fiction just long enough for them to escape back to the TARDIS. The Doctor intends to alert the Time Lords and have a barrier set up around the Land of Fiction; then, he will take Ace and Benny to 1976, where, according to the Writer, a man named Danny Pain saved the world from aliens. History has been changed once again, and perhaps this time, the Doctor will find the answers he seeks.