School is over, final exams are behind — and Dima was the first to think of waving away from home. The convictions of his elder brother Viktor about a serious attitude to the future life only more “warmed up” the four friends, and for the first time they went to Tallinn for the first time without the bored care of adults...

A black child rescued from by Russian sailors, whom they name Maksimka, having overcome his fear of white people, turns out to be an unusually smart and kind little boy. The rude drunken sailor Ivan Luchkin was especially attached to the boy. Once it was Maksimka who managed to save Luchkin from foreign recruiters of labor. For resourcefulness and courage, the young hero is enlisted in the team of the corvette and becomes the cabin boy of the Russian fleet.

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.