At six o'clock every day, without fail, with no excuses, Sam Vimes must go home to read Where's My Cow?, with all the right farmyard noises, to his little boy. There are some things you have to do.It is the most loved and chewed book in the world.

But his father wonders why it is full of moo-cows and baa-lambs when Young Sam will only ever see them cooked on a plate. He can think of a more useful book for a boy who lives in a city.

So Sam Vimes starts adapting the story. A story with streets, not fields. A book with rogues and villains. A book about the place where he'll grow up.