In 17th-century France, Father Urbain Grandier's protection of the city of Loudun from the corrupt Cardinal Richelieu is undermined by a sexually repressed nun's accusation of witchcraft.
Cardinal Richelieu and his power-hungry entourage seek to take control of 17th-century France, but need to destroy Father Grandier, the priest who runs the fortified town that prevents them from exerting total control. So they seek to destroy him by setting him up as a warlock in control of a devil-possessed nunnery, the Mother Superior of which is sexually obsessed with him. A mad witch-hunter is brought in to gather evidence against the priest, ready for the big trial.—Films Ranked
In 17th Century France, Cardinal Richelieu (Christopher Logue) is influencing King Louis XIII (Graham Armitage) in an attempt to gain further power. He convinces Louis that the fortifications of cities throughout France should be demolished to prevent Protestants from uprising. Louis agrees, but forbids Richelieu from carrying out demolitions in the town of Loudun, having made a promise to its governor not to damage the town.
Meanwhile in Loudun, the Governor has died, leaving control of the city to Urbain Grandier (Oliver Reed), a dissolute and proud but popular and well-regarded Catholic priest. He is having an affair with a relative of Father Canon Mignon (Murray Melvin), another priest in the town, unaware that the deformed, neurotic Sister Jeanne (Vanessa Redgrave), head of the local convent, is sexually obsessed with him. She asks for Grandier to become the convent's new confessor. Grandier secretly marries another woman, Madeline De Brou (Gemma Jones), but news of this reaches Sister Jeanne, driving her to insanity (this includes an attack where Sister Jeanne viciously attacks Madeleine when the latter brings back a book that the former had lent her, and Sister Jeanne accuses Madeleine of being a "fornicator" and "sacrilegious bitch", among other things).
Baron de Laubardemont (Dudley Sutton) arrives with orders to demolish the city, overriding Grandier's orders to stop. Grandier summons the town's soldiers and forces Laubardemont to back down pending the arrival of an order for the demolition from King Louis. Grandier departs Loudun to visit the King. In the meantime, Sister Jeanne is informed by Father Mignon that he is to be her new confessor. She informs him of Grandier's marriage and affairs, and also inadvertently accuses Grandier of witchcraft and of possessing her. Mignon relays this information to Laubardemont. In the process, the information is boiled down to just the claim that Grandier has bewitched the convent and has had commerce with the Devil. With Grandier away from Loudon, Laubardemont and Mignon decide to find evidence against him.
Laubardemont summons the lunatic inquisitor Father Pierre Barre (Michael Gothard), a "professional witch-hunter", whose interrogations actually involve depraved acts of "exorcism", including the forced administration of enemas to his victims. Sister Jeanne claims that Grandier has bewitched her, and the other nuns do the same. A public exorcism erupts in the town, in which all of the nuns remove their clothes and enter a state of religious frenzy. Duke Henri de Condé (actually king Louis in disguise) arrives, claiming to be carrying a holy relic which can remove the "devils" possessing the nuns. Father Barre "exorcises" the nuns with it. They appear as though they have been cured until Condé/Louis reveals the case allegedly containing the relic to be empty. Despite this proof that the possessions and the exorcisms are a sham, both continue unabated, eventually descending into a massed orgy in which the nuns sexually assault and desecrate a statue of Christ.
In the midst of the chaos, Grandier and his wife return and are immediately arrested. After being given a ridiculous show trial, Grandier is shaven and tortured although at his execution he eventually manages to convince Mignon that he is innocent. The judges, clearly under orders from Laubardemont, sentence Grandier to death by burning at the stake. Laubardemont has also obtained permission to destroy the city's fortifications. Despite pressure on Grandier to confess to the trumped-up charges, he refuses. He is then taken to be burnt at the stake. His executioner promises to strangle him rather than let him suffer the agonising death by fire that he would otherwise experience. However, the overzealous Barre starts the fire himself, and Mignon, now visibly panic-stricken about the possibility of Grandier's innocence, pulls the noose tight before it can be used to strangle the priest. As Grandier burns, Laubardemont gives the order for explosive charges to be set off and the city walls are blown up, causing the people to flee.
After the execution, Barre leaves Loudun to continue his witch-hunting activities elsewhere in the southwest of France. Laubardemont informs Sister Jeanne that Mignon has been put away in an asylum for claiming that Grandier was innocent (the explanation given is that he is demented), and that "with no signed confession to prove otherwise, everyone has the same opinion". He gives her one of Grandier's charred bones and leaves. Sister Jeanne, now completely broken, masturbates pathetically with the charred femur. Grandier's wife, having been released is seen walking away from the ruined city as the film ends.
