Ghost hunter Agatha Brewster returns
home to the United States and is met by news reporter Jeff Carter,
who is doing a story about the disappearance of her brother,
scientist James Brewster. Agatha learns from James's partner,
Dr. George Randall, that James has not really disappeared, but has
gone into hiding as a result of an experiment which turned him into
an ape-man. James still has his human intelligence, however,
and keeps himself locked in a cage with a gorilla in his secret
laboratory at Springfield, the Brewster estate.
Jeff is assigned a female photographer,
Billie Mason, who accompanies him on his pre-arranged interview with
Agatha at the mansion. During the interview, both Billie and
Jeff hear cries that sound like a gorilla. When Billie later
develops her photograph of Agatha, she sees the ape-like face of
James in the background. George refuses to help James obtain
the human spinal fluid which might restore his humanity, so James
murders George's butler and obtains the fluid from his body, forcing
George to inject him. The serum fails, but James is convinced
that he merely needs a stronger dose. George flatly refuses to
assist him again.
With the help of his gorilla, with whom
he can communicate, James goes on a murderous rampage to obtain more
spinal fluid. Jeff and Billie become suspicious of the
Brewsters after reading reports that ape hair was in the victims'
hands. While they investigate the Brewster mansion, James
tries to force George to inject him, but George angrily smashes the
vial of spinal fluid. James attacks and kills George, and
Agatha, who had rushed over to warn George, arrives too late to save
him. Billie is abducted by James, who takes her into his
secret laboratory, intending to draw her spinal fluid.
However, the gorilla becomes territorial and attacks James.
After killing the ape-man, the gorilla
comes after Billie, but Agatha provides the police with the clue to
the secret entrance to the laboratory, and the police kill the
gorilla in time to save Billie.
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