At his psychiatric institute, Dr. Charles Conway, John Carradine is surreptitiously tinkering with artificial glands in an attempt to achieve longevity; he works with his imposing minion Lobo, Tor Johnson, and his assistant Dr. Sharon Gilchrist, Marilyn Buferd. Conway receives 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. When Conway balks at Wright for bringing him a patient with living relatives, he confides in Conway that he plans to throw Grace's purse and bags 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, who famously delivers the line “Time for go to bed!”, discovers Frank Scott, Myron Healey roaming 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 over to the police, he offers him the chance to take part in his experiments.
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 Natalie Andries, Sally Todd, whose treatment regimen 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.
Jedrow is ordered buried alive by Lobo, 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 Dr. Conway's unethical deeds to the other patients. After a failed attempt to reveal Natalie's fate, he manages to show Grace and Danny what had happened to her, only to be 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 by Scott. 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 investigate the psychiatrist's practice. Dr. Conway evades arrest, but is murdered by Jedrow. Lobo kills Jedrow, but Houston's police backup arrives soon afterward, arresting Lobo and Sharon, and barely saving Grace from the procedure. The police go downstairs and find Danny's body, and then discover a menagerie of horrors, all failed subjects of Conway's longevity experiments. The police captain wonders, “Good Lord, what if they do live forever?”
