Plot
The Great God Om tries to manifest himself once more in the world, as the time of his Eighth Prophet is nigh. He is surprised, however, when he finds himself in the body of a tortoise, stripped of his divine powers, except for the ability to singe eyebrows with tiny thunderbolts.
In the gardens of Omnia's capital of Kom, he addresses the novice Brutha, the only one able to hear his voice. Om has a hard time convincing the boy of his godliness, as Brutha is convinced that Om can do anything he wants, and would not want to appear as a tortoise.
Brutha is gifted with an eidetic memory and is therefore chosen by Vorbis, the head of the Quisition, to accompany him on a diplomatic mission to Ephebe as his secretary. However, Brutha is also considered unintelligent, since he never learned to read, and rarely thinks for himself. This begins to change after Brutha discovers Ephebe's philosophers; the idea of people entertaining ideas they are not certain they believe or even understand, let alone starting fistfights over them, is an entirely new concept to him.
With the help of Ephebe's Great Library, and the philosophers Didactylos and his nephew Urn, Om learns that Brutha is his only genuine believer. All others either just fear the Quisition's wrath or go along with the church out of habit. After learning that Vorbis had facilitated the death of the missionary Brother Murduck to cover up his being mocked by Ephebian citizenry and to provide a casus belli for war against Ephebe, Brutha uses his memory to reluctantly aid an Omnian raid through the Labyrinth guarding the Tyrant's palace. Because of his authorship of De Chelonian Mobile (The Turtle Moves), which contradicts Omnian dogma about the shape of the Discworld, Didactylos is brought before Vorbis to face reprisal. However, seemingly conceding his previous views about the shape of the world and willing to write a retraction extolling Omnian interpretations, Didactylos escapes after hitting Vorbis with his lantern. Ordered by Vorbis to burn down the Library, Brutha memorizes many scrolls in order to protect Ephebian knowledge as Didactylos sets fire to the building to stop Vorbis reading its scrolls. Completely unrelated to the story, the Librarian of the Unseen University travels through L-Space to rescue several of the abandoned scrolls.
Fleeing the ensuing struggle in Urn's steam-powered boat, which is destroyed as the price for an earlier deal made between Om and the Sea Queen, Brutha and Om end up washed up on the desert coast. Trekking home to Omnia with a catatonic Vorbis, they encounter ruined temples dedicated to long-dead, long-forgotten gods, the faint ghost-like small gods yearning to be believed in to become powerful, the small-god-worshipping anchorite St Ungulant, and the human cost of Vorbis's plan of leaving caches of water in the desert to attack Ephebe. Realising his 'mortality' and how important his believers are to him, Om begins to care about them for the first time.
While Brutha, Vorbis, and Om are in the desert, the Tyrant of Ephebe manages to regain control of the city and contacts other nations who have been troubled by Omnia's imperialistic ambitions. Sergeant Simony, whose native Istanzia had been conquered by Omnia in his youth, brings Didactylos and Urn to Omnia to lead the Turtle Movement in a rebellion against the Church. However, Didactylos asserts that De Chelonian Mobile was meant to be a statement of facts rather than a revolutionary text.
On the desert's edge, a recovered Vorbis attempts to finish off Om's tortoise form (killing an ordinary tortoise by happenchance instead), knocks out and abducts Brutha, and proceeds to become ordained as the Eighth Prophet, elevating Brutha to archbishop to buy his silence. After Urn accidentally activates the hydraulic system which secretly operates the doors of the Great Temple, Brutha interrupts Vorbis's ordainment. As a result, Brutha is to be publicly burned for heresy while strapped on a heated bronze turtle but Om comes to the rescue, dropping from an eagle's claws onto Vorbis' head, killing him. As a great crowd witnesses this miracle they come to believe in Om and he becomes powerful again. In the ethereal desert, Vorbis learns to his horror that what he thought was the voice of Om was in fact his own voice echoing inside of his own head, plunging him into despair and leaving him unable to cross the desert and face judgement.
Om manifests himself over the citadel and attempts to grant Brutha the honour of establishing the Church's new doctrines. However, Brutha wishes to establish a 'constitutional religion' whereby Om Himself obeys Omnianism's new commandments and answers some of the prayers of his followers in exchange for a steady source of belief, believing that Om will lose his power again otherwise.
Meanwhile, Ephebe has gained the support of several other nations along the Klatchian coast and has sent an army against Omnia, establishing a beachhead near the citadel. Brutha attempts to establish diplomatic contact with the generals of the opposing army, wishing to stop the war and subsequent retaliation before it starts by surrendering and offering numerous concessions. Despite trusting Brutha, the leaders state they do not trust Omnia and that bloodshed is necessary. At the same time, Simony leads the Omnian military including Urn's 'Iron Turtle' war engine to the beachhead in order to fight the anti-Omnian alliance.
While the fighting occurs on the beachhead, Om attempts to physically intervene, but Brutha demands he does not interfere with the actions of humans. Om becomes infuriated but obeys Brutha, instead travelling to Dunmanifestin, where gods gamble on the lives of humans in order to gain or lose belief. While there, Om manages to unleash his fury, striking other gods and causing a storm that disrupts the battle. Eventually, he compels all other gods of the forces at the battle to tell their soldiers to stop fighting and make peace.
In the book's conclusion Brutha becomes the Eighth Prophet, ending the Quisition's practice of torture and reforming the church to be more open-minded and humanist (after Brutha appoints Simony as the new Head Exquisitor and Didactylos as a bishop), with the citadel becoming home to the largest non-magical library on the Discworld. Om also agrees to forsake the smiting of Omnian citizens for at least a hundred years. The last moments of the book see Brutha's death a hundred years to the day after Om's return to power, and his journey across the ethereal desert towards judgement, guiding the spirit of Vorbis whom he found still in the desert and upon whom he took pity. It is also revealed that this century of peace was originally meant to be a century of war and bloodshed which the History Monk Lu-Tze changed to something he liked better.
