The focus of the action is Walter Hartmann (Karl-Heinz Heß), who in the course of the 52 episodes works his way up from a motorized patrolman (as the so-called "white mouse") to a detective and is confronted with a wide variety of cases in the Hanseatic city of Hamburg. His teacher is the elderly inspector Koldehoff (Josef Dahmen), who heads the district's criminal police. Together with him and his colleagues from the Kripo Schlüter (Eckart Dux) or Bollmann (Günter Lüdke) and the district warden Kröger (Karl Heinz Kreienbaum), Hartmann solves every case, no matter how difficult. In the 4th season, Inspector Koldehoff is transferred, he becomes head of the robbery department. Commissioner Castorp (Heinz-Gerhard Lück) will be his successor. In the second season, Hartmann is allowed to take part in two police exchange programs and so he and his foreign colleagues solve five cases in France and two cases even in Japan.