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After escaping from a Capetown mental institution, psychopathic murderer Robert Griffin returns to London to seek revenge upon the Herrick family. Meanwhile, the unsuspecting Sir Jasper and Lady Irene Herrick entertain their daughter Julie's new beau, newspaper reporter Mark Foster at their home. After Julie and Mark leave, Robert meets with Jasper and Irene and accuses them of abandoning him when he was injured on an African safari. The Herricks profess their innocence, arguing that the native guides had told them that Robert was dead. Robert demands his share of the diamond fields discovered on the safari, but Jasper tells him that they were lost in a series of bad investments. When Robert threatens to sue the Herricks, they offer to share their Shortlands estate with him, but he suggests an arranged marriage between himself and Julie instead. Robert then passes out and the Herricks quickly realize that their old friend has become dangerously insane. After removing their partnership agreement from Robert's jacket, Jasper throws Robert out of the house. Later, the semi-conscious Robert is saved from drowning by a Cockney cobbler named Herbert Higgins. When Herbert attempts to blackmail the Herricks, Jasper calls the chief constable, Sir Frederick Travers, who dismisses Robert's claims against the Herricks and orders him to leave the district. On his way back to London, Robert arrives at the home of Dr. Peter Drury, a reclusive scientist who has invented a formula for invisibility. Drury agrees to inject Robert with the experimental formula, unaware that the murderer plans to use his newfound invisibility against the Herricks. Robert later forces Jasper to sign over his entire estate, then helps Herbert win his rent money in a dart game at a local inn. After Jasper promises Robert his daughter's hand in marriage if he regains his visibility, the madman returns to Drury's laboratory, where he sees the doctor restore the visibility of his dog Brutus through a blood transfusion. After Drury tries to call the police for help, Robert knocks the scientist unconscious and performs the transfusion himself, which leads to the doctor's death. The now-visible Robert sets the laboratory afire and escapes just as Mark arrives with the police. Returning to the Herrick estate, Robert assumes the alias of "Martin Field" and moves in with the Herricks. Herbert later arrives at the estate, and despite his feeble attempt to blackmail Robert, the murderer offers to pay his old friend 1,000 pounds to kill Brutus, who has followed Robert to the Herricks'. When Robert begins to lose his visibility while having breakfast with Julie and Mark, he lures Mark into the wine cellar, knocks the reporter unconscious and begins a second blood transfusion. Travers, however, arrives at the estate, having learned of Robert's presence from cab driver Al Prouty, and with the help of Herbert and Jasper, they break down the cellar door just in time to save Mark. Robert, in turn, is mauled to death by Brutus. After Mark recovers, Jasper declares that Robert, with his mind clouded by imaginary enemies, received his just desserts.