In one of his most successful portrayals of a "living dead" man, Boris Karloff plays John Ellman, an ex-convict who is framed by the mob for the murder of the judge who first put him away. Evidence proving Ellman's innocence arrives seconds after he is electrocuted. Officials allow Dr. Evan Beaumont (Edmund Gwenn) to experiment with putting a mechanical heart into Ellman. The device revives the dead man, but he has become a white-haired, monster-faced zombie who hangs out in graveyards and seeks revenge on the conspirators who framed him.
by Michael Betzold synopsis