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 trendy gathering in a London art gallery, naive French model Anne-Marie DeVernet (Penny Irving) is shocked to see that her photographer boyfriend has exhibited 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 as a sledgehammer-subtle cypher for the infamous Marquis de Sade), who offers to take the unsuspecting model to his isolated country estate to escape the scandal her now ex-boyfriend has caused her.
Unfortunately, Anne-Marie all too soon discovers that Mark is a procurer of young girls for questionable '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, replete 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.
Allmovie called it a "disturbingly effective horror film", writing that "Many viewers will be offended by the film's repressive right-wing tone, but its genuine scares and creepy atmosphere will outweigh its philosophical offenses for most horror fans."
The influential Halliwell's Film Guide described the film as a "low budget psychological horror that stylishly achieves its object: to disturb", and quotes Derek Elley in Films and Filming: "Shows that something worthwhile in the entertainment-horror market can be done for the tiny sum of £60,000".
David Pirie wrote in Time Out: "An above average sexploitation/horror that has been put together with some polish and care from a fairly original script. The film is dedicated ironically to all those who wish to see the return of capital punishment in Britain, and it's about a senile old judge and his wife who are so appalled by current permissiveness that they set up a gruesome house of correction for young girls. The only trouble is that the film undercuts its potentially interesting Gothic theme by some leering emphases, and the final result is likely to be seen and appreciated only by the people who will take the dedication at its face value."Barbara Markham ... Mrs. WakehurstPatrick Barr ... Justice BaileyRay Brooks ... TonyAnn Michelle ... JuliaSheila Keith ... WalkerDorothy Gordon ... BatesPenny Irving ... Ann-Marie Di VerneyRobert Tayman ... Mark E. DesadeIvor Salter ... JackKaran David ... KarenCelia Quicke ... DeniseRon Smerczak ... TedTony Sympson ... HenryJudy Robinson ... ClaireJane Hayward ... EstelleCelia Imrie ... BarbaraBarry Martin ... AlRose Hill ... Henry's WifeDave Butler ... Ticket Collector