[1] On the morning of the 30th anniversary of the Glorious Revolution of the Twenty-Fifth of May (and as such the anniversary of the death of John Keel, Vimes' hero and former mentor), Sam Vimes — whose wife is in labour with their first child — is caught in a lightning storm while pursuing Carcer, a notorious criminal who has murdered several watchmen, to the roof of the Unseen University's Library.

Vimes's first idea is to ask the wizards at the Unseen University to send him home, but before he can act on this, he is arrested for breaking curfew by a younger version of himself.

Incarcerated in a cell next to his is Carcer, who after being released joins the Cable Street Particulars (otherwise known as the Unmentionables), the secret police carrying out the paranoid whims of the Patrician of the time, Homicidal Lord Winder.

When he is taken to be interrogated by the captain, time is frozen by Lu-Tze, who tells Vimes what has happened and that he must assume the identity of Sergeant-At-Arms John Keel, who was to have arrived that day but was murdered by Carcer.

Vimes, taking command of the watchmen, successfully avoids the major bloodshed erupting all over the city and manages to keep his part of it relatively peaceful.

However, the barricades are gradually pushed forward during the night (by Fred Colon and several other simple-minded watchmen) to encompass the surrounding streets until Vimes finds himself in control of a quarter of the city containing most of its food supplies, dubbed "The Glorious People's Republic of Treacle Mine Road", with a still alive Reg Shoe as one of the leading figures.

Several policemen (the ones who died when the barricade fell in the original timeline) are killed in the battle, as is Reg Shoe; Vimes manages to fight off the attack until he can grab Carcer, at which point they are returned to the future and Keel's body is placed in the timeline Vimes has just left, to tie things up, as in the "real" history, Keel died in that fight.

Night Watch is the twenty-ninth novel in the comic fantasy Discworld series, written by Terry Pratchett, and the sixth to focus on the character of Sam Vimes.

"[4] Robert Hanks of The Independent drew attention to a "slight softening of the funny bone" and a "hardening of the issues" in the later Discworld books, commenting on a lesser amount of jokes per page in Night Watch.

[5] The New York Times's Therese Littleton praised the book as "transcend[ing] standard genre fare with its sheer schoolboy humour and characters who reject their own stereotypes".