Detective Katarina Kopeikina accidentally gets a package with someone else's letter. She decides to return it to its sender, Anna Furmanova, but she, it turns out, has died. Then Kata carries the letter to the addressee Boris Pleshnikov. However, even there a fatal surprise awaits: Pleshnikov’s wife is killed almost before her eyes, giving her an injection in the neck. Soon the widower is dealt with in the same way. Kata opens the letter. In it, Furmanova asks Boris to improve relations with her. A completely innocent request against the backdrop of so many murders! Katarina doesn't like all this very much, and she starts an investigation!..