Christmas is approaching and the chief of police in the town of Valleby intends to put on a Christmas play in memory of his parents, but a lot of crime gets in the way. He therefore enlists the help of two young detectives, Lasse and Maja. Lasse and Maja have their own detective agency that solves crimes. When the orchestra learns that the police chief has not gathered enough actors, they become increasingly suspicious of his amateurish methods.