lassov (Yul Brynner) is a Soviet spy. In France, Vlassov defects. He is whisked to the U.S.A., where Allan Davies (Henry Fonda) takes over the case. After polygraph tests and cross-examinations, Vlassov names several Western European agents who are also spying for the Soviets. Davies wants to take the listed agents into custody; meanwhile, those on the list start dying under mysterious circumstances.

Overview from themoviedb.org