The film is set in post-WWII Moscow. A young reconnaissance officer Vladimir Sharapov (Vladimir Konkin) returns from the war and is assigned for peacetime duty at the famous MUR (Moscow Department of Criminal Investigations). There he becomes part of a team lead by the brilliant, tough, no-nonsense homicide detective Gleb Zheglov (Vysotsky). The duo becomes embroiled in two seemingly separate investigations: that of a murder of an aspiring young actress Larisa Gruzdeva, and a hunt for a vicious gang of armed robbers that calls itself "Black Cat" and constantly manages to evade capture. While suspicion in Gruzdeva's murder initially falls on her estranged husband Dr Gruzdev (Yursky), it gradually becomes apparent that that the two cases are connected, as a Black Cat member known as Fox is implicated in the murder. As a result of Zheglov's successful high-stakes operation to capture Fox, Sharapov inadvertently finds himself undercover at the Black Cat's hideout, sparring with the gang's menacing leader The Hunchback (Dzhigarkhanyan). What ensues is arguably one of Soviet television's most memorable and suspenseful finales.
Much of the series revolves around the relationship of Zheglov and Sharapov. While the two become close friends and roommates, they also clash throughout the film. The source of the conflict is Sharapov's disagreement with Zheglov's "ends justify the means" approach to law enforcement. Zheglov thinks that "a thief's place is in prison, and the public couldn't care less how I put him there". To that end, Zheglov thinks nothing of using dubious tactics such as planting evidence to justify the arrest of a known mugger. Sharapov, on the other hand, considers that law is a higher value for its own sake and cannot be used merely as a tool. A tense conflict also arises when, in order to mislead Fox, Zheglov elects to continue to hold Dr Gruzdev under arrest even after it becomes clear that the man is innocent.