Sam Vimes can't tell what kind of day he's having. One moment he's fighting a ruthless murderer on top of the library of the Unseen University. The next, he's thrown back in time.

He meets his younger self, and sees a lot of people who, last time he saw them, were, well, older. To top it all off, he's been mistaken for his former commander, the man who taught him all about being a good Watchman. But the city's on the brink of revolt, there's a curfew, the police are corrupt, and that killer he was after in the future is with him here in the past, which is now the present...sort of. Now all Vimes has to do is figure out how to get back home—but first he has to change the outcome of a bloody revolution.

There's a problem, though: if he wins, he's got no wife, no child, no future...