Night Watch

This is one of the darker entries in the satiric Discworld fantasy series. Sam Vimes, Commander of the Ankh-MorporkWatch, is pursuing a psychopathic killer when both of them are transported back 30 years in the past, during a time of great unrest in the city. Posing as his old mentor in the Watch, Sergeant John Keel (who was murdered in this timeline), Vimes must minimize the bloodshed in a looming civil revolt, catch the killer, and ensure that his younger self, currently a rookie, becomes an honorable policeman.