The focus of the crime series is a Franco-German police unit headed by Steffen Herold. Leni Herold, the daughter of the experienced police officer, also belongs to the group of investigators working from Kehl. From France, Yves Kléber was sent across the Rhine to support the unit, which in exceptional situations can quickly become active on both sides of the border. Kléber and the old herald also have a long-standing relationship that began over 20 years ago and at that time went beyond professional life.