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.