Gisi, Manne, Bert and the other students from class 6b did not expect that their summer vacation would be so thrilling. It all started when they were invited to ride in a new electro-locomotive. Because they want to thank Mr. Selm, the class teams up to help him work on his new house and decide he needs a wading pool in the garden for his children. A lack of materials threatens this plan until the children discover an old abandoned house that might have what they need.
While exploring the hourse, the children discover several motorcycles in the basement - someone is clearly trying to hide stolen goods. What should they do about this crime?