Following the invention of a life-saving artificial heart, a test that accidentally kills a volunteer subject sends Dr. Henryk Savaard (Boris Karloff) to the gallows. Before dying, Savaard vows to avenge his death. So when a reporter who's investigating the mysterious suicides of six of the jurors follows the remaining principals in the case to a meeting at the late doctor's home, he finds Savaard alive and luring them to their deaths. But when cornered by the police, the death of his daughter (Lorna Gray) in all the confusion inspires the fatally wounded Savaard to use all of his remaining powers to restore her to life and finally destroy the mechanical heart.