Boris Karloff is back as Dr. John Garth and this time he is on trial for euthanasia/murder after he carried out a mercy killing on an elderly friend of his.
Garth reveals during the trial that he had been researching a cure for aging, but had not had time to perfect it before his friend’s pain became unbearable. Despite his pleas for mercy, the judge sentences him to be hanged in three weeks’ time.
As he awaits his execution, Dr. Garth is allowed to continue his experiments as his prison warden (Ben Taggart) and another scientist who is interested in his research, Dr. Ralph Howard (Edward Van Sloan) see the benefits it could bring humanity.
Then one night they use the blood of a recently executed murderous prisoner (Why murderous? Stay tuned!) to successfully develop a serum that will reverse the effects of aging and they decide to test it on Dr. Garth immediately prior to his execution. As he is being taken away to the gallows, however, the prison receives a call informing them that Dr. Garth’s sentence has been commuted to life imprisonment. At the same moment, the serum’s effect on his body causes Dr. Garth to collapse!
When Dr. Garth awakes in the prison medical ward, he discovers that the serum has reversed some of the effects of aging on his body, including the graying of his hair, the appearance of his face, and his physical fitness.
Encouraged by the success of his serum, Garth decides to perform another test of the serum, this time on Dr. Howard. As he is preparing Dr. Howard for injection, however, Dr. Garth is overcome by a sudden urge to kill, induced by the presence of the executed murderous prisoner’s blood in his system. Garth then strangles Dr. Howard, and an ill fated prisoner who enters the room at the wrong time…