An adventure film is shot in a seaside village on the border. The director of the film asks to find a good swimmer among the local teenagers. Romka Marchenko was invited as an understudy of the main actor, and gladly accepts. On the first day of filming, noticing the trail of a border transgressor on the beach, Romka reported him to the outpost. The film crew and the boy decide to help the police apprehend the criminal. Another story connected with the capture of the intruder is closely linked to the events on the set.