Pregnant Valerie (Kim Butcher) rushes home one night to her parents' house in tears, lets herself in and hurries up the stairs. Her father hears her and calls, but there is no reply. In her room, Valerie finds her Bible and scrabbles through the pages until she settles on one chapter, then sits down to write a note. When her parents venture up to see if she is all right, they have to break the door down when there's no sound and there they see the open window and through it, the body of Valerie who has committed suicide. What could have driven the young woman to this? Is it anything to do with a certain Father Meldrum (Anthony Sharp), the local priest?
For the horror films of director Pete Walker, writer David McGillivray took on various establishment targets at Walker's bidding: in House of Whipcord it was the law, in Frightmare it was the family and this time around it was religion. As with those previous two films, this was a bleak little example of the director's peculiarly provincial chillers, with heroines living with the realities of the permissive society in conflict with the disdain of the older generation who inevitably show themselves up to be far more corrupt, and corrupting, than the youngsters.
Not every member of the middle-aged and above is a villain here of course, but the ones who are really go to town with their perversity. Our heroine for this instalment is Jenny Welch (Susan Penhaligon - presumably Susan George was busy) who nearly gets knocked down by a priest in her first scene. He turns out to be a kindly, open minded young man of the cloth who Jenny knows from a while back, Bernard (Norman Eshley, best known for his appearances as the neighbour in sitcom George and Mildred). She has a coffee with him, then goes to tell her sister Vanessa (Stephanie Beacham) who she's just met.
Jenny stays in the flat above her sister's shop with her boyfriend Terry (Stewart Bevan), but her relationship with him has broken down and he's moved out today. Seeking solace, Jenny looks for Bernard to speak to at the local church, but oh dear, she actually ends up confessing to Father Meldrum who takes an unhealthy interest in her and tape records what she says for his future blackmailing purposes. This simple act of betrayal snowballs into a series of killings that see most of the cast kicking the bucket over the course of the story, and all because Meldrum's moralistic mania has pushed him over the edge into dangerous insanity.
Walker and McGillivray are careful to make sure they don't appear to believe all priests are homicidal nutters in waiting, but that's certainly the impression you take away from House of Mortal Sin. There are implications that Meldrum's mother (Hilda Barry), now a geriatric invalid, has sent him down this path, or it could be his housekeeper Miss Brabazon (Sheila Keith complete with one blacked-out lens in her spectacles), whatever, the cruelty starts early and doesn't let up. Jenny is forced into a "they won't believe me" plot, which unfortunately incapacitates her for too many of the later scenes, but the way the creepier, suffocating aspects of religion are brought out is bold and effective. As a whole, it's a callous, low budget and grey-toned work, but stays with the viewer longer than slicker horrors then or since. Music by Stanley Myers.