It’s apparent right from the start: Baby Billy has been born with a mustache. When asked what it means, the nurse replies that it depends on “whether it’s a good-guy mustache or a bad-guy mustache.” At first, it’s good. He becomes a toddler lawman, catching thieves with their hands in the cookie jar. But as his mustache grows and curls up at the ends, Billy becomes bad. Very bad. Throughout, the text relies on puns that the intended audience may not catch as Billy becomes a cat burglar and then a cereal criminal. What children will delight in, however, is Ang’s sturdy digital artwork, which gives her characters the heft of wooden toys even as they zip around the page.