Lasse and Maja are two kids who run a detective bureau. As the film starts, kids find a chest made by the composer Alexander von Broms in the local-church crypt, with a cryptic message attached to it. The chest appears to solve the church's financial problems; it's in urgent need of restoration. But when von Broms' relatives appear and claim that the chest is theirs, a conflict emerges. A few hours later, the chest gets stolen and Lasse and Maja tries to find it. The adventure begins in the city of Valleby with its surreal surroundings. There are puzzles to be solved and mysteries to be revealed and the whole family can join in.