In the life of Sergei Batashov, a young drummer of a pioneer organization, trouble comes. His father, an engineer at a secret factory, is arrested for losing documents. Guilty of this his stepmother marries and leaves Sergei alone. Using the trustfulness of the boy left to himself in the Batashovs' apartment, a criminal espionage organization was established. Introducing themselves as distant relatives of the boy, they use it for their own purposes, forcing them to acquaint them with his father's colleagues.