As Dr. Ernest Sovac (Boris Karloff) makes his way to the electric chair, he recalls the events that landed him on death row. After placing brain matter from a violent gangster into the mind of a dying teacher named George Kingsley (Stanley Ridges), Sovac watches his patient take on violent tendencies. When Sovac learns, through an almost-possessed Kingsley, about a hidden stash of mob money, the doctor entices the criminal in Kingsley to come out -- with murderous results.