Imprisoned for euthanizing a patient, Dr. John Garth (Boris Karloff) spends his time on death row trying to concoct a serum that will reverse the aging process. He injects himself with his potion and miraculously becomes younger, but, unbeknown to him, he is also tainted by the blood of a homicidal maniac that was used in the formula. After his sentence is commuted, Garth returns home to his loving daughter, Martha (Evelyn Keyes), but is soon compelled to go on a murderous rampage.