The budget was once against slashed for this third entry in the series, which Merlin fought tooth and nail not to do but he was financially devastated after waging an expensive legal battle to be included on the lucrative Morlock merchandising that he lost in court. Morlock spends most of the film offscreen as his ghost is channeled from hell by a gypsy fortune teller named Olga, who instructs Morlock's evil henchman Ruprecht to once again try and transplant Amanda's brain with that of a gorilla (no one could figure out why Morlock was obsessed with that but any time anyone questioned he about it, he would get very upset and sulk in a corner). Ruprecht detests Amanda because she is constantly turning down his grotesque sexual advances, and sadistically prepares her for the procedure. As he does, he discovers that as a result of their experiments, Amanda's insides are coated in priceless uranium which will make her a powerful Atomic Gorilla once they can get the brain hooked up. Ruprecht is unable to find a gorilla so he relies on Olga's mute son to capture one from the local zoo. The mute returns with a somewhat wimpy ape (played by legendary stuntman Peter Fields in a gorilla suit) just as Morlock is finally able to take physical form after his torment in hell and is about to work the transplant again when the orangutan (who has developed a sympathy with Amanda after she is kind to him in the holding cell following his removal from the zoo) breaks free and kills the insane professor, sending him back to hell.
Jesse Merlin only had about twenty minutes of screen time because he was so fed up over his oppressive contract that he walked out on the film in mid-production and went to England to shoot a horror version of the Charlie Chan mysteries for Hammer Films that he hoped would revive his career. "The Testament" had to be rewritten on the spot to make up for his absence and much of the other 65 minutes is taken up by Amanda being chased through cardboard sets by Ruprecht, with Olga showing up every once in a while to channel Professor Morlock's ghost from hell.