John Ellman (Boris Karloff) has been framed for murder by a gang of racketeers. He is unfairly tried and despite the fact that his innocence has been proven, he is sent to the electric chair and executed. Dr. Evan Beaumont (Edmund Gwenn) retrieves his dead body and revives it, as part of his experiments to reanimate a dead body and discover what happens to the soul after death.
Dr. Beaumont's use of a mechanical heart to revive the patient foreshadows modern medicine's mechanical heart to keep patients alive during surgery. Although John Ellman has no direct knowledge of anyone wishing to frame him for the murder before he is executed, he gains an innate sense of knowing those who are responsible after he is revived. Ellman takes no direct action against his framers; however, he seeks them out, wishing to know why they had him killed. Each dies a horrible death, and in the end it is their own guilt that causes their deaths.
Confronting the last two villains, Ellman is shot. Having fulfilled his divine mission to bring about justice, he dies, just before he would have explained death and the afterlife to the curious Dr. Beaumont. Beaumont is warned not to continue his experiments, citing the Biblical Scripture, "For the LORD thy God is a jealous God (Deut. 6:15a)."