The Dead Season (1968)

Soviet spy Ladeynikov learns that in one of the pharmaceutical centers in a small resort town works a former German war criminal, Dr. Hass, who is finishing the creation of a deadly chemical gas RH development that he began during World War II, experimenting on POWs. Since Ladeynikov doesn't know Dr. Hass's appearance, Soviet intelligence recruits an actor, Ivan Savushkin, who, during the war, escaped from a prison camp where Hass was testing his gas. Together, they must identify and stop him before he finishes and unleashes his weapon of mass destruction.