Terry Pratchett is known mainly as a writer of humorous fantasy—especially his Discworld series, of which this is the latest volume. But there’s more to the Discworld than humor. It started out as a series of parodies of well-known fantasy and science fiction writers, including Robert E. Howard, Anne McCaffrey, and J. R. R. Tolkien. But the Discworld is a huge canvas, large enough to contain serious themes amid the humor and parody. Night Watch explores those themes, in a way many readers of Prometheus will find worth reading.

The plot of Night Watch is about revolution. Pursuing a serial killer, His Grace Commander Sir Samuel Vimes, Duke of Ankh-Morpork, collides with some of the magic endemic to the Discworld—and he and his quarry fall into the past, the Ankh-Morpork of thirty years earlier. This is a much more brutal Ankh-Morpork, whose Patrician makes use of a secret police force to keep the population in terror. The strategy is about to fail, as public protest turns into revolution. Vimes remembers it, having lived through it as a young man. Now he lives through it again, as an older man, under an assumed name—in fact, the name of the man who taught the young Vimes his approach to policing, John Keel.

Pratchett doesn’t make a huge display of his political views. But the portrait of the head of the secret police, Captain Swing, will sound amazingly familiar to libertarian readers. At the abstract level, Vimes sees Swing as thinking not about how to deal with the way people are, but about how to make them be what they ought to be. At a more concrete level. Swing’s views have led to the passage of a Weapons Law, under which only soldiers and police can legally own weapons; and when ordinary citizens discover that criminals are willing to break that law, like any other, and arm themselves for their own protection. this leads to those ordinary citizens being arrested for possession of weapons, and sometimes for Assault on a City Official when they object to being arrested. This is the sharpest of Pratchett’s political points, but others concealed here and there in his story will seem equally congenial.

At a deeper level, Night Watch is about another congenial theme: the power to choose. We see Vimes, trapped in a confusing situation, having to make the gravest of choices: whether to become involved in the revolution he knows is emerging, whether to go on struggling to return to his own time and his wife and soon-to-be-born child, what to do when he finally confronts the killer he was pursuing. And throughout the story, we see him relying on his own mind and his own perception of reality as the basis for those choices.

People who have been reading the Discworld books aII along will meet a number of familiar characters here, seen much earlier in their lives, from Havelock Vetinari (in Vimes’s proper present. the Patrician of Ankh-Morpork) to Nobby Nobbs. The reader who starts out with this book will miss that particular pleasure—unless, as seems likely it sends them out to buy Pratchett’s 27 other books in the series, and learn who these people are, and come back able to get the rest of the jokes. In the process, they’ll find a surprising number of sympathetic passages. If Pratchett’s fiction has a moral point, it’s deep mistrust of those who prey on other people, especially if they cloak themselves in glamour and elegance to do so. We could hardly hope for a writer to share better chosen enemies with us.