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A curse pronounced upon land baron DeBerghmann has devastating long-range consequences for his twin sons Gregor and Anton. Twenty years later, Anton (Boris Karloff) has developed into a debauched and much-hated despot, disposing of his enemies by dumping their bodies into the titular "black room" on his estate. But when Anton's kindly, benevolent brother Gregor (also Karloff) returns home after a long absence, he is so beloved by the townspeople that Anton "graciously" gives up his title and estate to Gregor. Actually, Anton plans to kill his brother and take his place so that he can indulge in even more deviltry -- including the framing of young Lt. Lussan (Robert Allen), the sweetheart of the beautiful Thea (Marian Marsh), for the murder (committed by Anton) of Thea's father Col. Hassel (Thurston Hall). Ultimately, however, Anton's true identity is revealed by Gregor's faithful mastiff. Though reminiscent of an old folk legend, The Black Room was written directly for the screen by Arthur Strawn.