Dr. Savaard (Boris Karloff) creates a mechanical heart designed to bring life back to the dead. In order to test his machine, he puts a volunteer student to death. His assistant, Betty Crawford (Ann Doran), informs the police. Refusing to believe he can restore the student to life, the authorities place Savaard under arrest. The doctor is convicted of murder and sentenced to death by hanging. His faithful assistant, Lang (Byron Foulger), claims his body from prison and restores him back to life with the mechanical heart. Consumed with revenge, Savaard determines to murder everyone responsible for his conviction at the trial. He methodically murders six jurors under conditions resembling suicide.
The remainder of the trial forces are duped into entering Savaard's home, which is rigged with lethal booby traps. After several die by electricity and poison, Savaard’s daughter, Janet (Lorna Gray), arrives and attempts to free his prisoners, but she is electrocuted. While Savaard attempts to restore her back to life, the police burst in, mortally wounding him. After her life is restored, Savaard destroys his invention and dies.