In London, Gail Dunlip is brutally murdered when she opens a mysterious package containing an altered pair of binoculars that drive spikes through her eyes. Scotland Yard’s Superintendent Graham and Inspector Lodge question Gail’s roommate Peggy, but remain mystified by the gruesome attack, the third unsolved murder of a single, attractive woman within two weeks. Later Graham and Lodge receive a visit from crime journalist Edmond Bancroft, who accuses the police of incompetence. Upon inspecting the binoculars, Bancroft points out that they are similar to those found in Scotland’s Yard’s “Black Museum,” which houses unusual devices used in various murder cases. The inspectors then meet with Police Commissioner Wayne, who insists that they must get results from their investigation, as the public has grown alarmed by the grisly murders. Meanwhile, Bancroft visits his physician, Dr. Ballan, who cautions him against overwork. Ballan notes that Bancroft’s obsession with the details of each murder have resulted in the writer suffering a number of stress attacks, but Bancroft dismisses the advice. Later the doctor records his conclusion in Bancroft’s medical charts that the patient is in need of psychiatric care. On the way home, Bancroft stops at a small antique store run by an elderly woman, Aggie, from whom he regularly purchases unusual weapons for his own private “black museum.” Aggie offers Bancroft a peculiar curved knife and asks about the publication of his latest crime book. Bancroft returns home to his assistant Rick, who is at Bancroft’s bidding, unaware that Bancroft uses a combination of hypnosis and drugs to keep him under control. That evening Bancroft visits with his hired companion, Joan Berkley. When Bancroft admits that he cannot give Joan her usual allowance, she grows insulting and mocks Bancroft’s limp and intellectual pretensions. Declaring that she will no longer live in seclusion waiting for his visits, Joan leaves the apartment and spends the evening dancing at a nearby bar. Upon returning home, Joan prepares for bed and as she lies down, a bizarre figure standing behind her headboard sets off a miniature guillotine that beheads Joan. The man, whose face is distorted and strange-looking, packs the guillotine and head and flees from the building, witnessed by several of Joan’s neighbors alerted by her death scream. The next day, Graham attends Bancroft’s reception for the publication of his latest book. When the writer again accuses Scotland Yard of ineptitude, Graham surprises the author with his revelation that they have arrested the killer. Later at the jail, Graham and Lodge question the accused man, Tom Rivers, who corroborates numerous details of each killing, but whose strange eagerness makes Graham doubtful. Upon receiving Rivers’ medical record, which reveals he has spent long periods in mental institutions, Graham confides in Lodge that he suspects that Rivers knows about the murder details through Bancroft’s columns and is not guilty, only delusional. Graham orders that Rivers be held until he can be quietly transferred to a mental facility. That evening, after Bancroft departs his home, Rick hurries to the park to meet his girl friend, Angela Banks, who pleads to know when they can be open about their engagement. Bancroft returns to Aggie’s shop where she offers him a large pair of ice tongs, then demands an exorbitant fee. When Bancroft expresses surprise, Aggie presents him with a newspaper photo of the murder binoculars, which bear her private marking. Aggie points out that she sold him the binoculars and insists that as she unwittingly collaborated in his crimes, she should share in the wealth he has amassed writing about them. Bancroft agrees but when Aggie turns her back, attacks her with the ice tongs. The following day, Bancroft and Rick are working on Bancroft’s experiments with electricity when Ballan visits. The physician admits his suspicions that Bancroft is involved in the murders because of his extreme physical response to each crime and asks the writer to give himself up. Offering to consider the doctor’s advice, Bancroft maneuvers Ballan between two large electrodes, then electrocutes him. Bancroft then orders Rick to dump the body in a vat of acid. That evening Bancroft conducts a book signing and returns home early to surprise Rick and Angela in the museum. After asking Angela to wait in another room, Bancroft rebukes his assistant for violating their privacy and secret world. When Rick hesitates and appears uncomfortable, Bancroft gives him another round of drugs, reminding the younger man that through him he has proven that the myth of the dual natures of good and evil in people is true. Telling Rick that he hopes to allow him to explore the darkest part of the human soul, Bancroft then directs Rick on how to deal with Angela. Rick takes Angela to an amusement park and agrees with her suggestion that they ride through the tunnel of love. Upon exiting the ride, however, Rick has transformed into the distorted and strange figure from the drugs and, acting upon Bancroft’s command, stabs Angela, then flees through the park. Meanwhile, Bancroft is at Scotland Yard presenting his latest article on the unsolved murders to Graham, when word reaches them of the park murder and that the killer is trapped on the ferris wheel. Bancroft insists on accompanying the investigators. At the park a crowd has gathered under Rick, who clings to the ferris wheel. Upon spotting Bancroft, Rick calls to him that he has fulfilled the writer’s orders. Bancroft screams at the police to shoot Rick, but the younger man throws himself down upon Bancroft, stabbing him before dying.