[1][2] Dr. John Garth (Boris Karloff) is on trial for murder after performing a mercy killing on an elderly friend.

As he awaits his execution, Dr. Garth is allowed to continue his experiments, thanks to support from the prison warden (Ben Taggart) and another scientist who is interested in his research, Dr. Ralph Howard (Edward Van Sloan).

When Dr. Garth awakens 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.

As he is preparing Dr. Howard for injection, however, Dr. Garth is overcome by a sudden urge to kill induced by the presence of an executed murderer's blood in his system.

He returns home to live with his daughter, Martha (Evelyn Keyes), and continues his research on the anti-aging serum.