The book opens with Max writing invitations to his birthday sleepover, and feeling unsure about inviting the new kid, Sam, but his mom insists. Max can’t put his finger one what it is about Sam, just that there seems to be something strange about him. Throughout the story they never do come right out and say what the something is — the reader has to put the clues together. We learn he has a really good sense of smell, he’s not sure he can come to the birthday party because there’s a full moon that night, he likes his burgers really rare, after they change into pajamas that night he develops fangs and claws and really hairy hands, and when the moonlight shines on him, he runs out into the night. Though it never comes right out and say he’s a werewolf, the point is that though the boys are initially scares of his nighttime transformations, when they join him in his backyard adventures they all have a really good time and end up really liking him just as he is, not despite his weirdness, but because of it.