Robert Griffin escapes from a Cape Town mental institution after murdering two orderlies, carrying a painful hunger for revenge against the Herrick family. That same night, Sir Jasper Herrick Lester Matthews and his wife Lady Irene Herrick Gale Sondergaard scrutinize their daughter Julie’s new partner, Mark Foster, a determined journalist. When Julie and Mark eventually leave the family residence, Jasper and Irene are left alone with their unsettling guest, and Robert decides to pay them an unannounced visit. He confronts them with accusations that they abandoned him during a dangerous African safari, insisting they owe him not only a share of their discoveries but also a say in the fate of his life.
The couple insists they believed him dead and denies any claim to the diamond fortune tied to their safari. They offer Robert a stake in their estate, the Shortlands, hoping to placate him. They even float a disturbing bargain: marry Julie off to Robert to cement the deal. After a tense exchange, Lady Irene drugs the man and he slips into unconsciousness. Realizing their guest has spiraled into madness, the Herricks decide to steal back the partnership paperwork and push him further toward ruin instead of helping him. While rummaging through Robert’s things, they uncover the signed agreement and cast him out of the house. He nearly drowns in his fall, but is saved by Herbert Higgins Leon Errol, a Cockney cobbler with an uneasy sense of opportunism.
Herbert tries to use Robert’s information to blackmail the Herricks, but the plan stalls when the local authorities, led by Chief Constable Sir Frederick Travers Leyland Hodgson, step in. Travers rules Robert’s claims void and orders him to leave the jurisdiction. On his way to London, Robert crosses paths with the enigmatic Dr. Peter Drury John Carradine, a scientist eager to test a provocative invisibility formula on a living subject. The would-be avenger sees a perfect tool for his revenge and agrees to a deal, forcing Jasper to sign over the entire estate to him. In a swift, brutal move, Robert also helps Herbert win a high-stakes round of darts at an inn, a small act of gratitude that hints at the uneasy alliance forming between them. Jasper privately agrees to marry Julie to Robert if the latter ever regains his visibility.
Back at Drury’s laboratory, Robert [Martin Field] emerges from the shadows, taking residence at the Shortlands estate under a new identity. Drury demonstrates a chilling triumph: his serum can restore visibility, as shown when a dog named Brutus briefly reappears in the light after a transfusion. In a grim turn, Robert knocks Drury unconscious and uses the doctor’s blood for a transfusion on himself, triggering the fatal outcome that ends Drury’s experiments and his own liberty as the lab is set ablaze and he escapes into the night.
With his identity secured and the estate now under his control, Robert tries to consolidate power while also silencing his new ally, Herbert, whom he pays to disappear. He also commands Herbert to murder Brutus, the loyal dog who has followed him back to the estate. The tension rises as the breakfast table reveals Robert beginning to lose his visibility again, a telltale sign that his supernatural bargain may be unraveling. He lures Mark Foster into the wine cellar, where another transfusion is attempted using Mark’s blood, hoping to restore his hidden power. Sir Travers arrives, alerted to the troubling events at the Shortlands, and, aided by Herbert and Jasper, they burst into the cellar just as the transfusion nears completion, racing against time to save Mark’s life. In the final, brutal moment, Brutus confronts the would-be tyrant, and Robert Griffin is killed, his plan undone by the very mystery he sought to control.
