1978 year. The group of Major Ivan Petrovich Cherkasov begins a new investigation: a young gambler Kira Miller and her lover, Interior Ministry Colonel Potapenko, were killed at a dacha outside Moscow. It turns out that shortly before her death, Kira lost four hundred thousand rubles in an underground casino - "katrana". To find out all the circumstances of the case, Cherkasov decides to put Sonya Timofeeva at the gambling table. Her task is to get into the trust of the regulars of the katran and collect the information necessary for the investigation. At the casino, Sonya meets the pianist Boris, nicknamed the Musician, and the owner of the katran, Bella. At first, Sonya is lucky in cards, there is excitement. She has an affair with the Musician. Sonya does not have time to look back, as she is too keen on the bohemian life and wallows in debt ... Cherkasov worries about his ward.
