The story begins with a scientist creating a device shaped like a man that can be remote-controlled by a machine. The mechanical man possess super-human speed and strength. The scientist is killed however by a gang of criminals, led by a woman named Mado, who wish to get the instructions for building the mechanical man. The criminals are captured before they are able to get them and are brought to trial and condemned. Mado manages to escape and kidnaps the scientist's niece whom she forces to give her the instructions which she uses to build a mechanical man.
Chaney plays two roles: mad scientist Arthur Lamb and Lamb's "experiment", known only as the Ape Man. This hideous creature was the result of Lamb's attempts to transplant animal glands into human beings.
Lieutenant Commander John Byrne of the Naval Secret Service and his partner Pvt. Bill Riggers chase criminal Dr. Sin Fang, who is desperate to find a lost sacred seal. (The fifth and only surviving episode of a six-part serial about an evil mastermind inspired on Sax Rohmer's Dr. Fu Manchu.)
Tampering with life and death, Henry Frankenstein pieces together salvaged body parts to bring a human monster to life; the mad scientist's dreams are shattered by his creation's violent rage as the monster awakens to a world in which he is unwelcome.
A dark and stormy night. Pluto is spirited away to the spooky mansion of an evil genius for a mad transplant scheme to put his head on the body of a chicken. Mickey gives chase, but find himself threatened severely by the house and its denizens.
An ex-vaudeville actor is working as the assistant to a doctor who has Frankenstein aspirations. The ex-vaudeville actor kills the doctor and decides to assume the identity of the dead physician.
A brilliant but deranged neurosurgeon becomes obsessively fixated on a judge's daughter. With the help of an escaped criminal whose face he has surgically deformed, the mad man lures her, her father, and her fiancé to his isolated castle-like home, where he has created a torture chamber with the intent of torturing them for having 'tortured' him.
An insane surgeon's obsession with an actress leads him to replace her wounded pianist husband's hands with the hands of a knife murderer--hands which still have the urge to throw knives.
A crazed scientist invents a serum that induces a catatonic state in anyone who gets the injection. He uses the serum to paralyze his enemies, in order to bury them alive.
A plastic surgeon goes mad when he discovers that his wife have an affair. He fakes her death and disfigure her and locks her in the cellar.
University professor George Kingsley is struck by gangsters while crossing the street, leaving him with brain damage and one of the gangsters, Cannon, paralyzed. Kingsley's friend Dr. Sovac attends to both men, and when Cannon offers him a reward for aiding his recovery, Kovac transplants part of Cannon's brain into the dying Kingsley's skull, creating a dual personality.
Dr. Leon Kravaal develops a potential cure for cancer, which involves freezing the patient. But an experiment goes awry when authorities believe Kravaal has killed a patient. Kravaal freezes the officials, along with himself. Years later, they are discovered and revived in hopes that Kravaal can indeed complete his cure. But human greed and weakness compound to disrupt the project.
Dr. Bernard Adrian is a kindly scientist who seeks to cure a young woman's polio. He needs human spinal fluid to complete the formula for his experimental serum. Meanwhile, a vicious circus ape has broken out of its cage, and is terrorizing the townspeople. Can there be a connection?
A reporter sleuths the mystery behind an oft-married Viennese doctor whose wives met mysterious fates.
A fugitive doctor (Lionel Atwill) tries to raise the dead for South Sea Islanders.
A university chemistry professor experiments with an ancient Mayan gas on a medical student, turning the would-be surgeon into a murdering ghoul as part of a plan to steal his lover.
A mad doctor (Bela Lugosi) and his helpers (John Carradine, George Zucco) lure girls to his lab for brain work, to help his wife.
A fugitive, dangerous madman reaches an English village where he confronts his former partner who left him for dead in the jungle after their discovery of a diamond mine. When the former partner also claims to have since lost the mine and all its wealth, which he took all for himself, and though the partmer is still living in a state of luxury , the madman takes up an offer from a crazed scientist to make him invisible, something the scientist has already done with experimental animals, so that he can take revenge.
Dr. Hohner, theatre physician at the Vienna Royal Theatre, murders his mistress, the star soprano when his jealousy drives him to the point of mad obsession. Ten years later, another young singer reminds Hohner of the late diva and his old mania kicks in. Hohner wants to prevent her from singing for anyone but him, even if it means silencing her forever.
A scientist working on cures for rare afflictions, such as a bone softening agent made from molds to allow him to correct the spinal deformity of his nurse, finds the physical causes of lycanthropy in wolf-man Larry Talbot and of vampirism in Count Dracula, but himself becomes afflicted with homicidal madness while exchanging blood with Dracula.