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In the small French-Canadian town of St. Marc sur Richelieu, beautiful Cora Laurent, whose much older husband, Dr. Paul Laurent, is away on business, displays a keen interest in the town's new physician, Dr. Pearson, much to the dismay of her prudish sister, nurse Marie Corbin. Cora dismisses Marie's concerns as petty jealousy, because Marie had been engaged to Laurent before he married Cora. Pearson brushes aside Cora's advances, as he does those of pretty Denise Turner, the eldest daughter of the Turner family, in whose home he rooms. On the day that Laurent returns, Cora visits Pearson at home to show him a spiteful, anonymous letter she has received, which accuses her of having an affair with him. The letter, written in red ink, is similiar to one received that day by Pearson, but he assures her that it is of no consequence. The following day, Pearson shows the letter to Laurent, who professes not to believe the accusations and surmises that the writer is mentally ill. At the hospital, Pearson receives another threatening letter, ordering him to stop the affair. He then speaks with Jean Louis Gauthier, a young war hero whose prolonged hospital stay has made him paranoid. Pearson upbraids Marie for her care of Jean Louis, thereby increasing her dislike of the doctor. Soon after, hospital administrator Dr. Helier and accountant Higgins also receive letters accusing them of wrongdoing, and they ask Laurent to investigate. Laurent tells Pearson that Marie is the primary suspect, but also questions Pearson about his mysterious abandonment of his London practice two years earlier. Pearson refuses to divulge any information about his past, and then, while tending to Denise, discovers that she has a clubfoot. Pearson apologizes for his rude behavior and admits that he cares for Denise, but the next morning, is upset to discover that he has been sent another letter. The latest missive directs him to go to the church that evening, where the writer will be waiting. At the church, Pearson meets a frightened Cora, who received a similar letter. Laurent and Marie are also there, and the unhappy Pearson realizes that the "poison pen" writer has succeeded in making the townsfolk suspicious of everyone. Soon after, Pearson rejects Denise, telling her that he has personal reasons for not continuing their relationship. Tragedy strikes when Jean Louis receives a letter telling him that he has incurable cancer, and the youth commits suicide. During the funeral procession, a letter falls from the hearse, accusing the mayor of using the occasion to make an election speech. Police investigator Robert Helier, Dr. Helier's son, takes over the case, and Laurent explains that differences in the letters indicate that there may be two authors, who may be afflicted with folie à deux , a form of insanity in which two people share similar delusions. Robert and the mayor bring Marie in for questioning, and although she maintains her innocence, she is arrested. Later, Pearson, who has reconciled with Denise, takes her with him on a house call, and is irritated to learn that his patient is going to another doctor because she believes the gossip about him and Cora. On their way home, Pearson confesses to Denise that when he lived in London, his wife left him for another man. When she tried to return to him, Pearson rejected her and she committed suicide. After emerging from a deep depression, Pearson moved to Canada to seek a quiet life. On Sunday, a poison pen letter, declaring that the writer and Marie are not the same, floats down from the choir loft, and Robert is forced to release Marie. Laurent conducts an examination of the eighteen people in the loft, including Denise and Cora, and has them copy out several of the letters. After the ordeal, Cora warns Denise that Pearson will never marry her, and asserts that the rumors about their affair are true. Laurent then informs Pearson that the handwriting test proves that Denise is the culprit, which upsets Pearson. When Pearson confronts Denise, he sees a sample of her handwriting and realizes that Laurent lied, and also assures her that Cora was lying. Pearson goes to see Laurent, who confesses that Cora is guilty and that he has been trying to cure her of her mental illness. Cora is taken to the hospital, and when Pearson questions her, he realizes that the cruel and insanely jealous Laurent forced her to write some of the letters, but that he wrote the letter to Jean Louis himself. Pearson promises to help Cora, but as he leaves, he realizes that Jean Louis' mother has overheard their conversation. Pearson reaches Laurent's home too late, for Mrs. Gauthier has already killed the doctor with the same razor that her son used to commit suicide. Later, Pearson has re-established his practice, and he and Denise look forward to a secure future.