Dangerous Flight (1968)

Original title: Опасен полет

Professor Dimov is a diplomat who has been recruited as a spy by a foreign intelligence service. The suspicions fall on Dr. Belcheva. Major Kalinov is entrusted with the investigation of the case. He learns that she is Dimov's mistress. Paris, France. Dimov and Belcheva go out leaving a bag with secret documents in Belcheva's hotel room. Kalinov manages to retrieve the documents. After this incident Belcheva dies in hospital from poisoning. On board the plane Kalinov exposes the professor as a spy. Bulgarian counterintelligence officers arrest him.