Edward d'Eath, an Assassin and son of a down-and-out noble family, is determined to track down the lost heir to Ankh-Morpork's throne, and place him as leader, no matter how many people he has to kill.
Meanwhile, Captain Samuel Vimes, captain of the Ankh-Morpork City Watch, is confronted by new problems. About to be married to Sybil Ramkin, richest woman in Ankh-Morpork, he has to deal with the new recruits first: a dwarf, a troll, and a werewolf, representatives of ethnic minorities in the city. He also has to figure out who stole a mysterious device from the Assassins' Guild and solve a string of gruesome, seemingly random murders.
As the story progresses, it is made clear that d'Eath has stolen the 'gonne,' the disc's first firearm. He meant to use it to set the rightful king on the throne, but the device appears to have a strange mind of its own. After d'Eath takes it to Dr. Cruces along with his evidence of the king's identity, he is murdered by Dr. Cruces who then becomes a puppet of the gonne.
The Watch foil his attempt on the Patrician's life, losing Cuddy in the process, and Vimes and Carrot tail him into the sewers. After a brief struggle, Vimes manages to take the gonne and corner Cruces in his office in the Assassin's Guild. With Carrot's help, he resists the weapon and Carrot, learning of his heritage, kills Dr. Cruces with his distinctly non-magic sword. The gonne is destroyed, and Vimes takes over in the revived post of Commander of the Watch.
- En man på sin vakt (Swedish)
- The title of the book is a double pun, both on the professional standards of the Watch (a man at arms being a professional warrior) and on the lack of actual men in the Night's Watch (only Vimes and Colon can be classified as men, as Corporal Carrot considers himself a dwarf, Constable Cuddy is a dwarf, Constable Destrius is a troll, Constable Angua is both a werewolf and female, the Librarian is an oranutang, and Corporal Nobbs was disqualified from the human race for shoving).