Time is a resource that has to be managed. On Discworld, the Monks of History store time and pump it from the places where it's wasted—like underwater (how much time does a codfish need?) to places that need it—like cities, where there's never enough of time.
But the construction of the world's first truly accurate clock starts a race against, well, time, for Lu Tze and his apprentice Lobsang Ludd. Because it will stop time. And that will only be the start of everyone's problems. Thief of Time comes complete with a full supporting cast of heroes and villains, yetis, martial artists and Ronnie, the fifth Horseman of the Apocalypse (who left before they became famous).
