At his psychiatric institute, Dr. Charles Conway (John Carradine) is surreptitiously experimenting with artificial glands to try to create longevity; he works with his minion Lobo (Tor Johnson) and his assistant Dr. Sharon Gilchrist (Marilyn Buferd). Conway receives his test subjects through an associate, Dr. Loren Wright (Roy Gordon), who delivers patients seeking treatment for lesser conditions. After this, they are then taken into the operating room for Conway's illicit surgery.
Wright delivers his newest find, Grace Thomas (Allison Hayes), who is seeking treatment for depression. Wright confides in Conway that he threw Grace's purse into the bay, to fool family and the authorities into believing she had committed suicide. He then asks Conway for a demonstration of his experimental progress; Conway takes him down into the basement, where he introduces him to Harry Jedrow (Harry Fleer), his latest victim. Jedrow is clearly alive, but severely disfigured and in a vegetative state; this concerns Wright, who reveals that Jedrow's sister is currently seeking him out. Conway is furious, since none of his patients were supposed to have ties of any kind.
That night, Lobo discovers Frank Scott (Myron Healey) roaming around the grounds. Scott attempts to conceal his identity, but Conway quickly deduces that he is an escaped convict from his description in the newspapers, as well as a telltale tattoo on his wrist. Rather than turn Scott into the police, he offers him the chance to take part in his experiments. Knowing the odds are stacked against him, Scott accepts his offer.
Scott is introduced to Grace the following morning, along with the two other patients: Danny Green (Arthur Batanides), who is being treated for anger issues, and pretty young Natalie Andries (Sally Todd), whose treatment schedule for a nervous breakdown is nearing completion. After demanding Wright to make out a certificate of death for Harry Jedrow, Conway happily informs Natalie that one last treatment for her is all that's necessary. While the other patients sleep, Natalie is sedated, taken to the operating room, and given an artificial gland along with a high dosage of electricity. The procedure backfires, and she ends up a senile old woman. They hide her in a back room.
Lobo is ordered to bury Jedrow alive, but Frank Scott sneaks out to the burial site and opens the coffin. Jedrow rises out of it and escapes, and Lobo - not having been alerted - buries the casket. Sharon confronts Conway about his apparent affinity for Grace, and requests that she be made the next patient to be experimented upon. Meanwhile, Scott begins attempting to reveal to the other patients that Dr. Conway is carrying out horrific deeds to their friends. After a failed attempt to reveal Natalie's fate, he manages to show Grace and Danny what had happened to her, only to get caught by Dr. Conway and Sharon. They detain Scott and Danny and prepare Grace for surgery.
Danny helps Scott escape by distracting Lobo, who fatally shoots him before being knocked unconscious. Scott confronts Dr. Conway with Lobo's gun and reveals that he is not a convicted murderer; he is actually Lt. Mark Houston, an undercover police officer sent to the psychiatrist's business to investigate it. Dr. Conway evades arrest, but is murdered by Jedrow. Lobo comes in and kills Jedrow, but Chambers' police backup arrive soon afterward and arrest Lobo and Sharon, barely saving Grace from the procedure. The police go downstairs and find Danny's body, and then discover a menagerie of beastly men, all failed experiments of Dr. Conway.John Carradine as Dr. Charles Conway
Myron Healey (misspelled Myron Healy) as Mark Houston, alias Frank Scott
Allison Hayes as Grace Thomas
Marilyn Buferd (misspelled Marylyn Buferd) as Dr. Sharon Gilchrist
Arthur Batanides as Danny Green
Sally Todd as Natalie Andries
Tor Johnson as Lobo
Harry Fleer as Harry Jedrow
Roy Gordon as Dr. Loren Wright
Guy Prescott as Police Captain George Reagan
Raymond Guth (misspelled Raymond Guta) as Police Officer Miller
Paul McWillimas as Police Officer Ed
The film was completed in 6 days by at least May 1957.
In the United-States, it was distributed as a double-feature with The Beginning of the End (1957).
Marilyn Buferd was Miss America 1946.
The film was mocked on the television show Mystery Science Theater 3000.The Unearthly was originally released on DVD by Image Entertainment.
The MST3K version of the film was released by Rhino Home Video in April 2003. That DVD was recalled in March 2007, but then put back in print in June 2007.