Millie is twelve years old, and a schoolgirl detective. In this story, her younger brother Dave realises his new mobile phone is missing. Someone accuses his best mate Joe of stealing it, but Millie and Dave don't believe that. If the mystery is not solved, Joe will be branded a thief, and one or both boys may not be allowed to appear in Hilldown School's Winter Show. Will Badger the school bully be somehow involved? Might Boris the dog be able to help? And will Millie collect the clues and track down the thief in time? This is a stand-alone detective story, but there are brief references to Millie's first case: Millie the Detective and the Diamond Ring.