The life of Sergei Batashov, a young drum player at a pioneer organization, was bothered by a serious trouble. His father, an engineer at one secret factory, was arrested for the loss of the documents. His stepmother, who was guilty of what has happened to his father, finds another husband, leaving Sergey alone. Using the trustfulness of the boy left to himself a criminal-spy organization settled down in Batashov’s flat. Posing themselves as boy’s relatives they use him for their purposes coaxing him to acquaint them to his father’s co-workers. It took sometime before Sergey figured out who is who.