A parcel is delivered to Gail, a young blonde, who opens it to reveal a pair of binoculars. Her flatmate, Peggy, is curious about the sender. When Gail takes the binoculars to the window to check the view, there is a terrified scream, as Gail turns away with hands over her eyes, collapses and dies. The binoculars fall to the floor, revealing two blood stained spikes. Peggy exits in horror.
Later, Peggy is interviewed by Superintendent Graham and Inspector Lodge at Scotland Yard. Celebrity journalist and crime writer Edmond Bancroft appears. He has heard about this latest in a series of killings, and is keen to examine the lethal binoculars. Graham says they are similar to binoculars in Scotland Yard's notorious private 'Black Museum'.
Bancroft visits his doctor, Dr Ballan, and says he cannot rest until the killer is captured. Ballan observes that after each murder Bancroft is in a state of shock. Ballan later dictates notes on Bancroft, stating that he is in need of psychiatric treatment and should be hospitalised.
Bancroft pays a visit to Aggie's antique shop and buys a long knife. At his large home, he descends with his assistant Rick to his secret museum featuring weapons used to commit bizarre crimes. Bancroft claims his museum is better than that of Scotland Yard.
Bancroft visits buxom blonde Joan, who demands money and taunts Bancroft for being a cripple. In a bar, she dances provocatively to music from a jukebox. Returning to her flat and bed, she looks up, gasps, and sees a guillotine, and a figure with a hideously made-up face triggering it off. The blade falls, and her scream draws a gathering of neighbours. The crazed killer escapes through the crowd. Graham interviews the neighbours, who comment on the killer's bizarre appearance.
Bancroft meets Graham at his publishers' cocktail party, and Graham tells him that they have detained Tom Rivers, who has made a full confession of the killings. Rivers is soon revealed as a fantasist, but Graham decides to keep Rivers in jail in order to flush out the real killer. Bancroft reads about this 'confession' in the newspaper and calls at Scotland Yard, requesting to see Rivers. Graham tells him that Rivers has been transferred to a mental institution.
Rick sneaks out at night to meet his fiancée, Angela, in the park, and he tells her that his mind is controlled by Bancroft.
Bancroft returns to the antique shop, where Aggie confronts him with the link between weapons she has sold him and the killings, demanding £1200 for a pair of ice tongs as her price for silence. Bancroft kills her with the tongs.
Ballan visits Bancroft and tells him that he should be sanctioned. Bancroft stuns the doctor by triggering a laser beam and Rick is instructed to attach a chain to the body and immerse it in a large tub of simmering acid. When the chain is pulled up, only a skeleton remains.
At a bookshop, Bancroft signs copies of his latest book. Returning home, he goes to his basement museum and finds Rick embracing Angela, who is curious about Rick's work. After she leaves, Bancroft, angry that his private world has been violated, injects Rick with a drug to ensure that he obeys his commands in future. He informs Rick that he will inherit his house and 'black museum', and instructs him on what he is to do about Angela.
At the funfair, Angela and Rick ride in the Tunnel of Love. As the ride concludes, a crazed Rick pulls a knife and kills Angela, and flees to the Hall of Mirrors. With the police in pursuit Rick climbs the funfair Big Wheel. Bancroft is at Scotland Yard when Graham receives a phone message that the killer is trapped. Both arrive at the scene and police urge the crazed Rick to come down. Rick addresses Bancroft, who urges the armed police to kill him. They shoot to kill, and Rick jumps from a great height with the knife in his hand and plunges it into Bancroft's heart.
As the two bodies lie side by side, Superintendent Graham confirms that Scotland Yard have finally solved the case of the 'monster killer'.
