Terry Pratchett's Night Watch

On the anniversary of Ankh-Morpork's great uprising, Vimes is locked in a deadly struggle with a psychopathic criminal, and falls off a roof and back in time - right back to the revolution he remembers from his youth. Back in his rough and tough past without even the clothes he was standing up in, Vimes has to ensure that history takes its course so that the right future will emerge, and stay alive so that he can get back to it. He must track down a murderer, teach his younger self how to be a good copper, outmanoeuvre the vile Cable Street Unmentionables and their suspiciously psychopathic new Sergeant, and look after a bloody revolution.