The story follows a plot by a secret brotherhood, the Unique and Supreme Lodge of the Elucidated Brethren of the Ebon Night, to overthrow the Patrician of Ankh-Morpork and install a puppet king, under the control of the Supreme Grand Master (Vetinari's secretary, Lupine Wonse). Using a stolen magic book, they summon a dragon to strike fear into the people of Ankh-Morpork.  Once a suitable state of terror and panic has been created, the Supreme Grand Master proposes to put forth an "heir" to the throne, who will slay the dragon and rid the city of tyranny. It is the task of the Night Watch – Captain Vimes, Sergeant Colon, Corporal Nobbs, and new volunteer Carrot Ironfoundersson – to stop them, with some help from the Librarian of the Unseen University, an orangutan trying to get the stolen book back.  The Watch is in bad condition; they are regarded as a bunch of drunks who just walk around ringing their bells (it doesn't help that this is mostly true). The arrival of Carrot changes this; Carrot has memorised the Laws and Ordinances of the Cities of Ankh and Morpork, and on his first day tries to arrest the head of the Guild of Thieves for theft in Ankh-Morpork (the Thieves' Guild is permitted a quota of legally licensed thieving, a concept that the book of ancient Laws does not take into account). Carrot's enthusiasm rings with the feeling nagging at Captain Vimes; that the Watch should prevent crime. Vimes begins investigating the dragon appearances, which leads to an acquaintance with Sybil Ramkin, a breeder of swamp dragons. Ramkin gives an underdeveloped dragon, Errol, to the Watch as a mascot.  The leader of the Elucidated Brethren is initially successful in controlling the dragon, but he has not accounted for the dragon's own magic. The banished dragon returns, and makes itself king of Ankh-Morpork, keeping the head of the Elucidated Brethren, the now imprisoned Wonse, as its mouth piece; and demands the people of Ankh-Morpork bring it gold, for its bedding, and regular virgin sacrifices.  Vimes is imprisoned in the same cell as the Patrician, who has been leading a relatively comfortable life, with the help of the rats he uses as spies. The Librarian helps Vimes to escape and he runs to the aid of Sybil, who has been chosen as the first virgin to be sacrificed. The Watch's dragon, Errol, reorganises his digestive system to form a supersonic jet engine and fights the king, eventually knocking the king out of the sky with a shock wave. While a crowd attempts to close in on the king to kill it, Sybil tries to plead for it, while Carrot places it under arrest; but Errol lets the dragon escape, to be his mate (It turns out the "King" is a "Queen").

Terry's been watching Dirty Harry films! It's not hard to imagine Eastwood as Vimes, as the story sends up every film he's ever made, including Every Which Way But Lose and Any Which Way You Can  There's some other stuff about a dragon and a long lost king returning to lay claim to his kingdom but it's been done already in Wyrd Sisters. Lance-Corporal Carrot is cast in the same mould as Hrun and cohen but unlike them he's smarter, and yet more innocent too. The rest of the watch (being Fred Colon and Nobby Nobbs at this point) are used very much as light relief but in cunning ways, their self referential jokes about their class and job make for refreshing reading, however the real star of the book is errol the Swamp Dragon, he's by far the most interesting character and you spend half the book wondering when he's going to explode, which sadly never happens, but in a good way. The magic never lasts of course, but then what really does? Luckily the whole concept of the watch is revisited later in a much better and funnier novel.