This is the twenty-ninth Discworld book, and it features the Ankh-Morpork city watch. Mostly, though, it focuses on Sam Vimes, who is sent back in time with a criminal named Carcer due to a lightning storm over the Unseen University. They arrive a few days before a revolution that Vimes participated in as a young recruit to the watch. In this new timeline, Carcer kills young Sam’s mentor, John Keel, and Vimes ends up taking his place.
The city during the revolution is very different from the Ankh-Morpork of the series’ present day. The patrician of the time employs secret police called Unmentionables, and the people are scared and angry.
At the end of the book, Vimes’ son is born in a sequence I found very touching. This is definitely one of my favourite books in the series.