This classic thriller takes on a sinister air when the mysterious Dr. Garth (Boris Karloff) kills a man while experimenting with a serum intended to alter the aging process! Sentenced to die, Garth goes to prison where he continues his bizarre work with Dr. Howard (Edward Van Sloan), the prison doctor. Suspense lingers when, on the day of his execution, Garth inoculates himself with a new serum made from the blood of a murderer! Just as he slips into unconsciousness, word is passed that his sentence has been changed to life imprisonment. When Dr. Howard sees Garth recover, looking years younger, he asks for the serum. The spine-chilling climax builds to a shocking end when Garth turns on the unsuspecting Howard with cold-blooded murder in his eyes!