Conducting weird scientific experiments, crazed Dr. James Brewster, aided by his colleague Dr. Randall, has managed to transform himself into a hairy, stooped-over ape-man. Desperately seeking a cure, Brewster believes only an injection of recently-drawn human spinal fluid will prove effective. With Randall refusing to help him, it falls to Brewster and his captive gorilla to find appropriate donors.
Famed psychic and ghost hunter Agatha Brewster (Minerva Urecal) returns home to the U.S. and is met at the docks by a gaggle of press wanting to interview her. Young news reporter Jeff Carter (Wallace Ford) catches her interest though as he says he is researching a story about the disappearance of her brother, the scientist James Brewster (Bela Lugosi). Agatha was unaware that her brother was missing and agrees to arrange for Carter to visit her at her home for an interview. Agatha immediately goes to see her brother's research partner Dr George Randall (Henry Hall) where she learns that James has not really vanished but in fact has gone into a self-imposed exile as the result of a disastrous experiment. In order to try and prove his evolutionary theories Dr Brewster had injected himself with an experimental serum to turn himself into a 'missing-link' between man and ape. The serum had succeeded in transforming James into an ape-man but with no way of turning himself back into a man the scientist has retained his intelligence but is now trapped in the body of a hideous man-beast. George tells Agatha that her brother keeps himself locked in his secret laboratory at Springfield, the Brewster estate, with a gorilla in a cage as he desperately seeks a remedy to his problem.Meanwhile at the newspaper office Jeff Carter is assigned a keen, female photographer Billie Mason (Louise Currie) to accompany him to the Brewster place for the pre-arranged interview with Agatha Brewster. During the interview both Jeff and Billie hear cries that sound like a gorilla. Agatha dismisses their suggestions but Billie thinks she sees something and takes another photo of Agatha. When the duo return to the newspaper offices Billie develops the photos she took at the Brewster place and sees the ape-like face of James Brewster in the background of one of them.James secretly goes to visit his colleague George Randall to tell him that he has made a breakthrough and believes that a possible cure for his condition lies with human spinal fluids, but when he realises how James means to obtain them George refuses to help James, so James in a rage murders George's butler (George Kirby) and obtains the spinal fluid from his body. James forces his reluctant colleague to inject him with the spinal fluid. The serum does not work but James is convinced that it is merely not a large enough dose to restore his humanity. George flatly refuses to help the murderous James obtain any more spinal fluid. So, back at his lab James (who can communicate with his gorilla in guttural grunts and cries) hatches a plan to send the gorilla out to obtain specimens for him to extract the vital spinal fluid from. The two go on a murderous rampage killing and killing again so that James can build up his stores of spinal fluid.Jeff and Billie become suspicious of the odd Brewster siblings when they read the press reports of the killings all of which state that the victims were found to be clutching ape hair in their dead hands. While the two newshounds are investigating the Brewster mansion, James again visits George to try and force him to inject him with the spinal fluid he has obtained, but George angrily smashes the vial of fluids. Enraged, James kills George moments before Agatha arrives – who has rushed over to warn George of her brothers intentions. Returning to the Brewster mansion James surprises Billie and takes her down to his secret lab intending to draw her spinal fluid. However, the gorilla becomes territorial and attacks James. After killing the ape-man the gorilla goes after Billie but Agatha provides Jeff and the police with the clue to the secret entrance to the lab and the police shoot the gorilla before it can harm Billie.