When Charlie's best friend Mike Blackburn's little sister eats his homework, Mike wants revenge. And Charlie knows he must be present to witness the act. After all, what are friends for? "I'm going to change her into a frog," Mike declares, waving a magic wand. This is a good idea. If Charlie had a sister, he'd want to change her into a frog. But something goes awry. Instead of changing Mike's sister into a little green fellow with warts, Charlie Fogalman suddenly isn't quite Charlie Fogalman anymore. He has a bit more fur-and a tail! "Don't tell me you've changed me into a dog." "Okay, I won't tell you." But the proof is in the drool. Mike knows he needs to change Charlie back, but how? What went wrong, and how does he change him back without making the situation worse? In the meantime, where does Charlie live? Not at the Blackburn's house, where the parents are allergic to dogs. And not at Charlie's house, where no one would believe it's really him underneath all the fur. Add to this, a teacher who just might be a Russian spy, an overly zealous dog catcher, and a school bully, and Charlie has plenty to occupy his canine brain. Eventually, the boys discover that the bad spell involved the twisting of light as it passed through Mike's science project, and they find a way to twist it back. But not before they put a bully, a spy, and a dogcatcher in their place.