Told in flashback as Dr. Ernest Sovac (Boris Karloff) is marched into the gas chamber, Black Friday concerns kindly college professor George Kingsley (Stanley Ridges), who is seriously injured when he is caught in the middle of a gangster shootout. Kingsley's best friend Sovac performs an emergency "brain-ectomy", replacing Kingsley's gray matter with that of dying gangster Red Cannon. Though the operation is successful, the mild-mannered Kingsley occasionally lapses into Cannon's more brutal personality, and it is during one of these spells that he reveals the existence of a cache of stolen money. Hoping to use these ill-gotten funds to finance his neurological research, Sovac hypnotizes Kingsley into "becoming" Cannon, and while thus entranced the poor fellow commits several murders, including the elimination of his chief rival, mobster Eric Marnay (Bela Lugosi). Ultimately, Sovac is forced to kill Kingsley/Cannon "for the good of mankind", which brings us full circle to Death Row.
~ Hal Erickson, All Movie Guide