British Captain Terence Stevenson (Robert Donat) accepts an assignment even more dangerous than his everyday job of defusing unexploded bombs. Fluent in Romanian and German and having studied chemical engineering, he is parachuted into Romania to assume the identity of Captain Jan Tartu, a member of the fascist Iron Guard. He makes his way to Czechoslovakia to steal the formula of a new Nazi poison gas and sabotage the factory where it is being manufactured.