Horrible Harry thinks he's the world's second-best detective (second to Sherlock Holmes, of course), but the rest of the kids in Room 3B aren't as sure. So he's determined to prove himself by solving the latest mystery at South School: a cafeteria contest has just begun, and Harry wants to figure out how to win it. He knows Mrs. Funderburke is using a mathematical code to decide each day's winners, but can he crack the code before Mary tattles on him? Or will his private-eye days come to a horrible end?