James Rankin (Boris Karloff) is a novelist fascinated by the 20-year-old case of the "Haymarket Strangler." He discovers that Styles, the man executed for the crimes, was really innocent, and that the blame actually belongs with Dr. Tenant, the man who performed the autopsies on the murder
victims. Rankin has Styles' grave unearthed and finds the doctor's missing scalpel, the murder weapon. When he picks it up, however, he becomes possessed with the murderous thoughts of the doctor. To his horror, Rankin discovers that he
is Tenant and that a wave of amnesia had blotted out the last
20 years from his memory. He goes on an uncontrollable spree of bloodshed, p...