Mr. and Mrs. Keller from Hamburg are going to spend a hiking vacation in the Black Forest, as well as going on a day trip to Strasbourg, a nearby French town. Sigrid Keller is a psychologist and Dieter Keller is a teacher. However, when a murder occurs at the hotel, Dieter's past is revealed and he becomes a suspect. Detective Eva Baumann investigates the murder together with her partner Ruediger Schulz. However the kitchen help, Lieselotte, knows exactly who is responsible for the murder: the witches from the nearby lake, Mummelsee.