On trial for murder after mercy-killing an elderly friend, Dr. John Garth explains he was pursuing a cure for aging but could not complete it before his friend's suffering became unbearable; the judge sentences him to be hanged in three weeks, a fate he accepts with quiet resolve.
While awaiting execution, he is allowed to continue his work thanks to the support of the warden, Warden Thompson, and another physician eager to see the theory tested, Dr. Ralph Howard. They devise a bold plan: using the blood of a recently executed prisoner, they create a serum that reverses aging and decide to test it on Garth just before his scheduled hanging. Just as the noose looms, a call arrives that his sentence has been commuted to life imprisonment, and at that moment the serum’s effects trigger a collapse in his body.
When he awakens in the prison medical ward, the transformation is striking: the graying hair retreats, the lines smooth away, and his vigor returns as if time itself has rewound. Buoyed by this miracle, he presses forward with another test, this time aimed at the immune system of Dr. Howard. As he prepares the injection, a surge of uncontrollable rage, summoned by the executed man’s blood, overtakes him and he strangulates Dr. Howard. A wandering prisoner bursts in, and in the ensuing struggle, that man also dies. The authorities, finding the two bodies and Garth’s apparent amnesia about the incident, conclude that the wandering inmate killed Howard and then attacked Garth.
Propelled by a newfound reputation as a savior and rejuvenated energy, Garth returns home to live with his daughter, Martha Garth, and resumes his anti-aging research with renewed purpose. He seeks out three aging friends to become his experimental subjects; one of them, Victor Sondini, initially resists, but a visit from Garth later persuades him to change his mind. The moment Garth is about to administer the serum, the blood-memory of his first victim floods him again, and he murders Sondini just as the others watch in horror. Terrified by his own actions, he then travels to see another friend, George Wharton, to confess and secure his consent for a final test—only to strike before help can arrive, killing Wharton as well.
As bodies mount and Garth’s behavior becomes increasingly erratic, Martha grows suspicious and confronts her father. He pleads with her to leave, but she refuses and his panic erupts into renewed aggression; she faints, and he uses the opportunity to escape. The chase culminates at the very prison where his ordeal began. The warden admits him, but Garth lunges toward the armed guard at the gate. The guard fires, and as he sinks to the ground, he reveals in his final confession that he chose death over continuing to harm others, admitting that he had effectively killed himself to stop the blood‑driven urge from consuming him.
