Late summer has arrived in Valleby, but everything is overshadowed by bad news. The small village's police station is threatened with closure, something that neither Lasse and Maja, nor the police chief want to happen. They must quickly figure out a way to save it. When veteran Granat's auctions come to visit, our heroes Lasse and Maja see their chance to show what Valleby and its police force are all about. With five newly installed alarms, and round-the-clock monitoring of the auction's valuables, LasseMaja's detective agency will make sure that nothing goes wrong. But all security is jeopardized when the notorious thief Konstskuggan turns out to be on the loose. The art shadow is known for being able to steal anything, and when the Valleby residents' auction items start disappearing one by one, she immediately becomes the prime suspect. The neighboring town's police chief Gabriella interferes in the investigation and nothing is as it usually is. How will things go for the chief of police in Valleby? And will they catch the art thief?