The film opens during a night-time thunderstorm when a frightened, confused and disheveled young woman runs down a country road and is picked up by a trucker. The film then shows through flashbacks how the young woman came to be in such a situation.
While at a gathering in a London art gallery, naive French model Anne-Marie DeVernet (Penny Irving) is shocked to see that her photographer boyfriend is exhibiting a recently-shot photo where she is seen being arrested by the police for public nudity. Humiliated, Anne-Marie dumps the photographer but soon finds solace in enigmatic fellow partygoer Mark E. DeSade (Robert Tayman), who offers to take her to his isolated country estate to escape the scandal her now ex-boyfriend has caused her.
Unfortunately, Anne-Marie soon discovers that Mark is a procurer of young girls for 'moral correction' by his sadistic mother, ex-reform school matron Margaret (Barbara Markham). Years earlier, Margaret was brought to trial when her corrupt reign over a girl's reform school led to the suicide of a young French girl under her charge (although in truth, Margaret murdered the girl and made it look like a suicide). Found not guilty but dismissed from her job in disgrace, she seduced the High Court Judge who heard her case (Patrick Barr). The judge, critical of the 'permissive society' of the England of the 1960s and 70s, nevertheless left his wife for Margaret, who bore him a son (Mark) who worked with her to turn their mansion home into a secret illegal prison for 'morally corrupt' and 'delinquent' young women, complete with a group of tough female wardens (led by Pete Walker regular Sheila Keith) who administer a harsh regime of corporal punishment upon their prisoners. However Mark and the now retired, blind and senile judge are oblivious to the fact that Margaret is in fact using the prison to torture and ultimately execute these young women upon them gaining three 'demerits' during their incarceration.
Anne-Marie soon falls foul of Margaret's cruelty as she reminds the evil matron of the charge she killed and whose death cost her her career and reputation. Meanwhile, Anne-Marie's concerned flatmate Julia (Ann Michelle) and Julia's boyfriend Tony (Ray Brooks) track down Mark, who has now discovered the full extent of his mother's murderous deeds at the prison after seeing her minions dispose of a prisoner's corpse.
Anne-Marie makes multiple escape attempts, but is recaptured every time. Her friends eventually find the prison, but too late to save her - she has been hanged after earning a third 'demerit'. As the police arrive Mark confronts his mother and is killed by her. Margaret, knowing the game is up, then kills herself with the same noose she set up for Anne-Marie. The judge and his wife's henchwomen are arrested, and the surviving prisoners are freed.
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