A religious sect known as The Brethren has taken control of widow Birdy Wemys (Ann Todd) and her unstable son Kenny (Tony Beckley). Filled with a religious mania from their teachings, Kenny Wemys is compelled to kill in the name of God, recording the killings to play back on audio tapes he mixes with the religious rants of the sect’s leader – The Minister (Patrick Magee).
Kenny is a swimming pool lifeguard by day and a security guard by night and uses both these occupations to further his third career as a serial killer. After killing his victims, he drives round in his Ford Escort van disposing of the bodies.
Birdy takes an instant dislike to Brigitte (Madeleine Hinde) the local district nurse assigned to care for the frail old woman. Brigitte is genuinely troubled by the Brethren, particularly the minister’s rule forbidding the use of medicine since Birdy is a diabetic and closet insulin user.
Brigitte’s sister Paddy (Suzanna Leigh) is a chain-smoking journalist encouraged by her sibling to write an expose of the Brethren. She poses as an unwed mother-to-be in order to join the fold but her appearance seems to bring out long-repressed desire in Birdy and when the minister finds out she’s been having ‘foul thoughts’ he orders Birdy to fast – effectively issuing a death sentence to the diabetic.
A desperate rush to administer her an insulin shot ensues. Paddy tries but is inadvertently stopped by Kenny, who locks her in the cellar. Finally standing up to the bullying minister Kenny storms upstairs to administer the shot, but he’s too late to save his mother and – quite literally – exerts some biblical vengeance on the minister.
Released in some markets as Beware My Brethren.