This is the twenty ninth Discworld book, and it features the Ankh-Morpork city watch. Mostly though it features Sam Vimes, as he is sent back in time with a criminal called Carcer due to a lightning storm over the Unseen University. They arrive a few days before a revolution that Vimes was a part of when he was a young recruit to the watch. In the 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 books present day. The patrician of the time has 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.