Thud! recap

As Ankh-Morpork approaches the anniversary of Koom Valley, the ancient battle between dwarfs and trolls, a militant dwarf leader is found murdered in a mine dug beneath the city, and the evidence seems to point at a troll. Commander Vimes must solve the killing before it sparks open war between the city's dwarfs and trolls. Investigating, he clashes with secretive deep-down dwarf zealots, follows a trail hidden in an old painting of the battle, and comes to suspect the murder was committed by dwarfs to conceal something. During the case an ancient dwarf spirit of vengeance, the Summoning Dark, marks and enters him, urging him toward murderous rage. Racing to reach Koom Valley itself before rioting begins, Vimes masters the thing inside him through his own inner discipline as a policeman, and uncovers the long-buried truth of the original battle, defusing centuries of hatred. Throughout, he clings to his rule of getting home to read his son's bedtime story, which becomes the anchor that saves him.

Previously, in earlier books: Sam Vimes rose from a drunken captain to Commander of the Ankh-Morpork City Watch and, by marriage to the aristocratic Lady Sybil, Duke of Ankh. He rebuilt the Watch into a large, mixed force of humans, dwarfs, trolls, a werewolf, and others, working alongside Captain Carrot, the honest human raised by dwarfs, and Sergeant Angua. In the course of his career he has repeatedly refused to let politics, patriotism, or privilege excuse a crime, and he distrusts his own capacity for anger. Recently he and Sybil became parents to a son. As this book opens, Vimes is an established, still deeply reluctant, member of the city's establishment, and a devoted, if awkward, new father determined to be present for his child.

The anniversary of Koom Valley, the ancient and much-mythologized battle between dwarfs and trolls, is approaching, and old hatreds are running high in Ankh-Morpork. A militant dwarf agitator who has been stirring anti-troll feeling from a mine beneath the streets is found dead, and the early signs seem to blame a troll. Commander Vimes is handed a case that could tip the whole city into ethnic violence, and given very little time to solve it. Amid the mounting pressure, he holds fast to one unbreakable personal rule: to be home every evening at six to read his infant son a bedtime story.

At Koom Valley, Vimes uncovers the secret the murdered dwarf died to suppress: the legendary battle was not simply a massacre but the site where a dwarf king and a troll king had come to make peace, a truth recorded and hidden long ago. This revelation gives both peoples a shared history other than slaughter and begins to defuse the feud. Vimes rejects and expels the Summoning Dark, the vengeance-spirit that tried to make him a killer, by holding to the incorruptible 'watchman' inside himself that refuses to cross that line. The conspiracy among the deep-down dwarfs is exposed and its members brought to justice, and the vampire recruit Sally proves herself alongside Angua. The threatened riots never come. Vimes returns to his family, having kept his promise to be home in time to read to Young Sam, the small ritual that stands, in the end, as the measure of the man.