The town of Valleby celebrates its 500th birthday in a few days! However, the celebration may be interrupted by a thief whose hallmark is... a scorpion. Will the young detectives Lasse and Maja track him down and save the party? Another hard nut to crack ahead of the detective couple. Someone is clearly very keen that the free candy party everyone in Valleby is waiting for doesn't happen. A telescope goes missing from the Science Academy, fireworks set for a show flash prematurely, and a live scorpion goes missing from a jewelry store. Maja and Lasse have several suspects. They do not even rule out that the priest may be guilty - after all, it was on the church tower that the clues were found. They are assisted in their investigation by a police officer and a retired thief, Lillie, who lives in a retirement home.