Boris Karloff plays Gregor de Berghmann and his twin brother, Anton, Czechoslovakian barons who have inherited their family estate. Twenty years after their father's death, Anton has returned from a long absence to find his depraved brother reigning as a despot, murdering his enemies and disposing of them in his castle's "Black Room." Because of their love for Anton, the locals implore Gregor to relinquish the estate to his benevolent twin. However, having lived under the specter of a curse that foretells of his death at the hand of Anton, Gregor kills him and dumps his corpse before assuming his identity in order to fool the townspeople and pursue a romance with the beautiful Thea Hassel (Marian Marsh). But when he kills Thea's father to keep the ruse a secret, fate deals the last hand in a strange - and deadly - end for Gregor in the Black Room.