Matthew's American cousin, Sam, has come to London to live with the family. Sam is a charismatic, funny kid, but can he be trusted to be a reliable friend? Matthew and his "mates" decide that Sam must undertake a challenge in order to prove himself: He must start off his new year at the school posing as a girl. It turns out that Sam makes a great girl. He fools everyone at Bradbury Hill School, stepping over lines and blurring boundaries of all sorts. And the longer the prank goes on, the more outlandish the repercussions.