|Olivia de Havilland||Terry/Ruth Collins|
|Lew Ayres||Dr. Scott Elliott|
|Thomas Mitchell||Lt. Stevenson|
|Lela Bliss||Mrs. Didriksen|
|Lester Allen||George Benson|
|Charles Evans||Dist. Atty. Girard|
After a doctor is murdered, Lt. Stevenson questions the man's girlfriend, Terry Collins . Sensing that she's keeping a secret, Stevenson confronts Terry in her home, where he meets her identical twin sister, Ruth (also de Havilland). Both women appear exactly alike, sometimes even posing as each other. However, when twin expert Dr. Elliott analyzes the sisters, he finds that one twin is normal, while the other is psychotic -- but which one?