The noble family de la Porte lives at castle Medan. After the lost heir August returns unexpectedly, countess Herm, mother-in-law of August’s younger brother Johann, fears for the inheritance of her daughter. Together the two women antagonize the brothers. During a brawl, Johann gets shot by accident. Countess Herm persuades August to leave his home for good. Beforehand he hides his papers of identity in the secluded great hall.
Now Johann’s oldest son would be the next rightful heir, but due to his physical and psychological deformity he is locked away to make room for his younger brother Gert. Years later, the dying August, who now resides in Chicago, confesses to his daughter Agga their true identity. At his request she travels to castle Medan to find the hidden papers and take possession of her legacy. On her arrival the countess already senses her true intention. She instructs her nephew Gert to court Agga for the protection of the estate. However, she falls in love with neighbour Magnus and rejects Gert.
Hence countess Herm and Gert forge a plan to unleash his brother, who is kept at the castle like an animal, on the uninvited guest. Because she is denied entrance to the great hall, Agga voluntarily sleeps in the nearby brown room, which is supposedly haunted. In the meantime, countess Herm frees the “maniac”. Her scheme shows success, but after his attack he flees into the depths of the castle. On the next morning, the terrified Agga asks Magnus to guard her at night. Following a repeated assault of her beastlike cousin, both of them escape to the great hall.
In search of her deranged nephew, countess Herm also enters the brown room, where she’s promptly killed by him. The hunt for the killer ends at the castle-tower, from where he leaps to his death. Along the way, Agga and Magnus investigate the great hall and discover the missing papers in a secret compartment. Finally, servant Tobias clears up the intrigue and the two lovers vow to "forever banish the forces of darkness from castle Medan."