Condemned to death for a mercy killing, Dr. John Garth ( ) continues to experiment in prison to develop a serum that will put at end to the aging process -- vowing to complete it . On the eve of his execution, he offers himself as guinea pig for his youth serum, which has recently been mixed with the blood of an executed psychopath. Miraculously, Garth grow younger before the astonished eyes of kindly prison physician Ralph Howard ( ). Alas, the serum has murderous side effects, which Howard discovers only as Garth strangles him to death. Pardoned from Death Row thanks to a script contrivance, Garth spends the rest of the film trying to carry on his humanitarian work despite embarrassing lapses into homicidal mania. Many observers regard as the best of Karloff's "Mad Doctor" series for Columbia.