It's late summer in the small town of Valleby, but the calm is disrupted by bad news. The police station is under threat to be closed, which neither Lasse, Maja nor the chief of police wants. They must find a way to solve the situation.
Lasse and Maja find an old mysterious coffin that turns out to belong to the eccentric family von Broms. A centuries-old family fights flare up again and suddenly the coffin gets stolen. Who wants to steal it and why?
The sleepy little town Valleby is getting ready for the cafe owners Dino Panini's and Sara Bernard's wedding, and our popular young detectives Lasse and Maja stand ready to ensure that nothing will go wrong. But when the wedding couple's respective relatives arrive, they prove to belong to rival mafia families.
Around the time that the chief of police is elected the Most Popular Citizen of Valleby, a thief and impostor is released after two decades in jail and returns to the town with a revenge plan in mind.
LasseMajas detektivbyrå was the 2006 julkalendern broadcast 1–24 December 2006 in Sveriges Television. The TV series, which was based on the book series LasseMajas detektivbyrå by Martin Widmark and Helena Willis, takes place in Lasse's and Maja's home town Valleby. It was recorded in Sveriges Television studios during the spring of 2006.