Plot
The Great God Om tries to manifest himself once more in the world, as the time of his Eighth Prophet is nigh. 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. Despite his amazing memory Brutha is illiterate, 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, 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 reason 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. 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.
On the desert's edge, a recovered Vorbis attempts to finish off Om's tortoise form, knocks out and abducts Brutha, and proclaims himself 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 but Om comes to the rescue, dropping from an eagle's claws onto Vorbis' head, killing him. The great crowd witnesses this miracle and comes to believe in Om, making him 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.
Ephebe has allied with 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 before it starts by surrendering. 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.
Om attempts to physically intervene in the battle, but Brutha demands he does not interfere with the actions of humans. Om is infuriated but obeys Brutha, and instead travels to Dunmanifestin, where gods gamble on the lives of humans in order to gain or lose belief. Om unleashes his fury on the 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 aftermath Brutha becomes the Eighth Prophet, ending the Quisition's practice of torture and reforming the church to be more open-minded and humanist, 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. A hundred years to the day after Om's return to power Brutha dies. In the afterlife he finds the spirit of Vorbis and, taking pity on him, guides him to his judgement. It is 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.
