Research results of enormous value are stolen from the experimental laboratory of a chemical company in the fictitious town of Bärenfurt in the GDR and seized at the Berlin sector border. The trail leads to the agent "Haas" at the Bärenfurt municipal theater. Captain Jentsch is at the theater to uncover his identity and arrest him. A murder takes place during the "Fiesco" premiere. Many people are suspects, including the head of the theater, Born, who is engaged to the actress Vera and has a conspicuously apolitical attitude. It is no easy task for the security officers to distinguish between reality and appearance in this milieu. Through precise work, they succeed in unmasking the perpetrator and arresting him after a chase through the theater.