Biographical film about the composer Rimskiy-Korsakov. Belongs to the gallery of costume historical and biographical films of the postwar cinema of the Soviet Union. The film tells about the last two decades in the life of Russian composer.

A finding of an old briefcase in a ruined house in 1970s leads to a treasure, hidden in the 1920s.

The teenager Misha Polyakov, being with his mother in a Ukrainian village on vacation with his grandmother and about to return home to Petrograd, offers his best friend Genka to go with him. Suddenly, a white gang of Nikitsky bursts into the village and attacks Misha’s house, where Commissioner Polevoy lives. The purpose of Nikitskiy (aka Nikolskiy) is a dagger located at Polevoy. Saving the commissioner, Misha learns from him the history and secret of the weapon, and receives it for storage with a request to solve the secret. Having returned with adventures to Petrograd, Misha Polyakov with friends begins to unravel the riddle of the dagger, in the handle of which is encrypted text.

Soviet spy Ladeynikov learns that in one of the pharmaceutical centers in a small resort town works a former German war criminal, Dr. Hass, who is finishing the creation of a deadly chemical gas RH development that he began during World War II, experimenting on POWs. Since Ladeynikov doesn't know Dr. Hass's appearance, Soviet intelligence recruits an actor, Ivan Savushkin, who, during the war, escaped from a prison camp where Hass was testing his gas. Together, they must identify and stop him before he finishes and unleashes his weapon of mass destruction.