A mad scientist uses an ape to murder his enemies.

In Paris, in the 1890s, Police Inspector Bonnard is stymied by a brutal killing on the Rue Morgue. Three months later, Yvonne, a cabaret performer, is killed in a fashion similar to the Rue Morgue murder. Police find a bracelet with tinkling bells on Yvonne's arm, and in her room, photographs of her jealous stage partner Rene and her boyfriend, university student Georges Brevert. One of Bonnard's men notes that the bracelet's clasp, once locked, cannot be opened, and is meant as a symbol of eternal love. When neighbors claim they saw the shadow of a man escaping over the roof, Bonnard suspects the athletic Rene, until a spurned and jealous wardrobe mistress inadvertently provides him with an alibi while trying to prove his guilt. Bonnard then turns his attention to Brevert, but Sorbonne psychology professor Paul Dupin and Dupin's fiancée and laboratory assistant, Jeannette Rovere, provide Brevert with an alibi. Dupin introduces Bonnard to a fellow professor, a guest lecturer from the zoo, Dr. Marais, who trains mice to respond to bells and has theories about the killer instinct in animals. Later, a mysterious assailant enters an apartment through a rooftop window and kills a painter's model, Arlette, who wears a tinkling bracelet exactly like Yvonne's. In their search for a strong and agile suspect, Bonnard and his men check out circus performers, but evidence points back to Dupin when they find a cameo in Arlette's room, which he claims was bought for Jeannette and stolen from his apartment. At the university, Marais lectures about an unresponsive deaf-mute named Marie, who turned hostility inward after suffering her husband's rejection. Later, Camille, a young woman living in Dupin's boardinghouse, is killed, and the police capture Dupin on the roof. Dupin protests that he was chasing the killer, but when the police find other incriminating evidence, Bonnard arrests him and refuses to believe that someone is framing him. As with the other victims, Camille is found wearing a bell charm. Still baffled by the killer's ability to escape from the victims' apartments, Bonnard hires a circus acrobat to find a means of escape from Arlette's room. When the acrobat fails, Dupin suggests to the incredulous Bonnard that the killer is an animal. Later at a waterfront dive, Jacques, a brute who labors for Marais, kills a sailor who teases him about an ape he brought from Malta. Meanwhile, Marais offers Jeannette a job studying the behavior of animals in the zoo and introduces her to a caged ape that responds to her show of affection. After she leaves, Jacques and Marais discuss how the ape accidentally killed the first victim on the Rue Morgue, but has since been trained by Marais to kill at his behest. Jacques taunts Marais by saying that Jeannette will reject him, just like the other women Marais had the ape kill. In his house on the zoo grounds, Marais finds Jeannette studying a portrait of his late wife, who he claims committed suicide. Urging her to forget Dupin, Marais kisses Jeannette, but she cringes. Then, realizing that the portrait shows fear in Mrs. Marais' eyes, Jeannette correctly guesses that bars on the windows of the room were meant to imprison the woman, not keep her safe from zoo animals. At Jeannette's rejection, Marais becomes resentful, but gives Jeannette a tinkling bracelet, claiming that it is from Dupin. Seeing Marais' hostility, Jeannette has another insight, that he, like Marie, has developed a neurosis in response to rejection. After locking Jeannette in the bedroom, Marais proceeds to the ape's cage, but the animal has killed Jacques and escaped. As Marais searches, the ape breaks into a dressmaker's shop and kills the proprietress. Meanwhile, at the police station, Bonnard wants Dupin to sign a pre-written confession, but Dupin, seeing the victims' bracelets on his desk, recalls how Marais trained his mice with bells. He suggests that Marais could train an ape in the same way, but Bonnard refuses to consider the theory. Then, a policeman reports the dressmaker's murder and brings in witnesses who saw the ape and a well-dressed man go down a manhole near the crime scene. Bonnard, Dupin and several policemen race to the zoo and Marais' home, where Marais has sent the ape to break into the bedroom window and kill Jeannette. Although the bracelet on her arm tinkles, the ape gently carries the fainted Jeannette out of the room, to the roof and into a tree. Seeing the police, Marais releases a lion from its cage, but the police shoot it, grab Marais and set up a safety net. Marais orders the ape to kill, but instead, it drops Jeannette onto the net. When the police then shoot the ape, it kills Marais in the fall to its death. Later, at the station, Bonnard cuts the bracelet off Jeannette's arm and closes the case.