As Commander of the City Watch, Sam Vimes is one of the happiest, wealthiest, and most powerful men in Ankh-Morpork. And he owes all his success to his mentor, John Keel, who taught him all he knows nearly thirty years earlier. Today, as Vimes chases a dangerous murderer through the streets, both men are sucked through a time portal and land in the Ankh-Morpork of thirty years ago. It’s bad enough that Vimes is stuck in one of the darkest periods of the city’s history—but the situation is made much, much worse when the criminal kills John Keel before his time. While writing a time-travel novel, many well intentioned writers fail to come up with a convincing theory for how to send their characters to another era; Pratchett avoids this trap entirely by intentionally putting forward the least convincing, but most entertaining, argument I’ve ever read. Science-fiction purists may have a hard time swallowing his theory (which includes monks with brooms, quantum physics, and the Baked-Bean Tin of Universal Oneness), but any other fan of the Discworld will enjoy this City Watch novel as much as the rest of Pratchett’s series.