Newly ordained as a Catholic priest, Donissan Gérard Depardieu is assigned to a quiet rural parish under the care of the seasoned mentor Menou-Segrais Maurice Pialat. The young priest wrestles with deep-seated doubts about his calling and has begun to punish himself through ritual flagellation in a bid to prove his worth. When he confides his feelings of inadequacy to his elder, the elder priest offers a compassionate assessment: it is not too late to choose another path, yet he also sees in Donissan a substantial gift—one that could become a force for good if guided by faith rather than fear.
A parishioner named Mouchette Sandrine Bonnaire, the 16-year-old daughter of a brewer, comes to seek aid from one of her lovers, a marquis named Cadignan Alain Artur. She wants to flee to Paris, but Cadignan explains that he cannot offer much assistance due to his own financial troubles. He does, however, allow her to stay the night. In the morning, she toys with his shotgun, which is loaded, and Cadignan dies. She slips away, cleansing the blood from her clothes as she goes.
Mouchette then visits another lover, a married physician named Gallet Yann Dedet. After they share a moment of intimacy, she reveals that she is pregnant and that Cadignan’s death has been ruled a suicide. Gallet reluctantly tells her that he will not help her with an abortion and that Cadignan cannot be blamed for the circumstances.
Back at the parish, Menou-Segrais grows uneasy about the growing discomfort surrounding Donissan’s unorthodox vocation, and he sends the younger priest to lend aid in a neighboring village. On a walk through the fields at dusk, Donissan is approached by a mysterious horse-trader who appears to be the incarnation of Satan. This figure cannot break Donissan’s resolve physically or spiritually, but he claims that the young priest possesses a rare and troubling gift: the ability to look into the souls of others.
Donissan faints and later awakens in the morning to find Mouchette wandering in the fields again. He tells her that he can see her life and thoughts clearly and urges her to repent for the sin she has committed against Cadignan. She returns home and, consumed by guilt, ends her life by cutting her throat with a razor. Donissan, terrified of the consequences yet driven by compassion, carries her battered body to the church and lays it before the altar in a public act of intercession, hoping to salvage her soul.
The event catalyzes a crisis in his career. The authorities move him to another parish, seeking to calm the scandal and the unease among the faithful. Yet as word spreads, people begin to regard him as a holy man, even as he remains internally tormented and spiritually uncertain. A farmer from a nearby village asks him to come to his dying young son. Donissan arrives with reluctance, almost ready to abandon the task, but he feels the weight of expectation on him. He enters the boy’s room, lifts the body for a moment of prayer, and, miraculously, the child’s eyes open.
The emotional and mental strain intensifies as Donissan confronts the limits of his faith and the demands placed upon him by those who seek miracles and guidance. He endures a nocturnal assault by Satan and begs God to spare him if there is any purpose left for his suffering. He survives the ordeal, resumes his duties at the confessional, and continues to wrestle with his vocation while offering solace and mercy to those who seek him. When Menou-Segrais eventually visits the confessional, the two men share a quiet, final moment of understanding. After the last penitent leaves, Menou-Segrais discovers Donissan dead, having borne the trials of his faith to the very end.
