While Lea can enjoy her summer holiday by the sea, Leo has to visit a castle in Upper Bavaria with his parents. And all because of his middle ear infection! But then a girl who looks exactly like Lea appears in a castle gate. Leo wants to see her, steps through the door and - suddenly finds himself in the Middle Ages! But Lea introduces herself to him as Käthe, daughter of a cooper from Burghausen. What does that mean? To Leo's horror, a return to the present is out of the question for the time being, because the wondrous castle gate has disappeared. At least Käthe helps Leo to get along in the Middle Ages. But then she is arrested because a brooch of the princess Elisabeth is found on her. Desperately Leo thinks about how he can help her.