Two friends on the trail of the Art Shadow Thief, who is involved in the theft of valuable items. Little detectives are trying to find out the truth and help a friendly commissar. Another movie from the series of Lassego and Mai adventures.

The story of one of the series of films about two young detectives takes place in a small Swedish town of Valleby. Lasse and Maja will try to help their friend the policeman solve the mystery. The matter becomes harder and more dangerous with every moment. How will the attempt to capture the Shadow of Art end? One thing is certain - nothing is as it seems, and appearances are wrong. Young detectives must use their cleverness and the ability to logically combine minor facts and outwit dishonest adults.