Horrible Harry thinks he's the world's second-best detective–second only to Sherlock Holmes, of course. But the rest of the kids in Room 3B aren't so sure. So he's determined to prove himself by solving the latest mystery at South School–how to win the new cafeteria contest. He knows the cafeteria lady is using a special mathematical code, but can he crack the code before his classmate Mary tattles on him again? Or will the case go cold right before Harry's eyes?