Summer holidays: fishing, riding in a truck, ploughing through whitewater. 11-year-old Jonas expects lots of fun and action during his school break. But his family, scattered between Slovakia and Germany, haven’t really spoken to each other since his father died. So, this summer, Jonas's mother Beate has prescribed him a healthy sea climate at Aunt Anne’s, instead of going to Slovakia as they used to do.

Jonas is stunned. He promptly packs his bags and sets off, on his own, to visit his cool grandpa Bernard in Slovakia. When he arrives, everything seems to be the same as ever, only grandpa has changed. Without a wife or job, the old railway worker is not in the best of spirits. 

Jonas knows that something must be done, and it’s all up to him to do it. He teams up with Alex, a tomboy who becomes his best friend, and together they develop a series of criminally good ideas. The consequences are soon felt – Bernard has a mysterious date coming up; the police are at his heels, and the children have to cope with a failed business idea. There is a lot to straighten out before everyone can trust each other and be friends again. In the end, nothing will be as before – it will be much better.