Convicted murderess Valerie Carns (Ann Blyth) is being transported to Norwich to be executed when a flood strands her and her guards at a convent hospital. Nurse Sister Mary (Claudette Colbert) becomes convinced of her innocence and sets out to find the real killer.

As rains flood Norfolk County, England, Sister Mary Bonaventure is busy running the hospital ward at the Our Lady of Rheims Convent with unfailing correctness. When Nurse Phillips rebels against Mary's exactitude, the nun berates herself for being inflexible and reveals to the Mother Superior that she has not felt peace since she discouraged her sister from marrying a corrupt man, and her sister committed suicide. Soon after, the roads are washed out, and police Sergeant Melling is forced to sequester convicted murderess Valerie Carns and her guard, Pierce, in the convent until they can reach the prison to carry out Valerie's death sentence. The townspeople have also crowded into the convent seeking shelter, and although they protest Valerie's presence, Mary tries to comfort the desperate young woman. Valerie slowly warms to Mary and informs her that she is innocent of poisoning her brother, the ailing pianist Jason Carns. The head of the hospital, however, Dr. Edward Jeffreys, informs Mary of the facts of the case: he was Jason's physician while Valerie was caring for her brother, and heard her state that she wished Jason was dead. At the trial, his testimony and other circumstantial evidence suggested that Valerie had administered a fatal overdose of her brother's medicine on the day he died. Mary, who understands feeling guilt and dismay over a loved one's death, feels drawn to Valerie and believes in her innocence. That night, Mary researches the trial with the help of her friend, Sister Josephine. They leave a photograph of Jason on a table, and when Jeffreys discovers his ill, distraught wife Isabel staring at the picture, he rips it up and gives her a sedative, warning her not get involved. Valerie later begs Mary to bring her fiancé, Sidney Kingham, to the convent. The Carnses' faithful half-wit servant, Willie, finds a rowboat to transport Mary to Sidney's town, and she goes even though she knows it is forbidden for her to help Valerie. Along the way, Willie reveals that Jason was a sadist, and that Willie stole one of his letters to punish him for mistreating Valerie. When they find Sidney, Mary convinces him to believe in his girl friend's innocence and delivers him to the convent, where he embraces Valerie passionately. The next morning, Melling discovers that Willie has destroyed the rowboat so Valerie cannot be taken to the prison, and presses charges against Mary for not stopping him. Against the Mother Superior's orders, she gives Willie's stolen letter to Valerie, and although it is clearly a love letter from a married woman to Jason, Valerie cannot guess who the writer might be. After an unknown assailant strikes Willie over the head that evening, the Mother Superior finds and burns the letter, stating that Mary's stubbornness is only hurting others. In the next room, Isabel asks Jeffreys if he retrieved her letter, and he tells her that if she keeps talking about her "dreams" he will have her committed. The phone wires are fixed, and Sidney, realizing that Valerie will soon be brought to the prison, begs Mary to help them. She at first refuses, but when Josephine brings more press clippings about the trial, they notice that the pharmacist, Abel Harmer, was inconsistent in his testimony. Jeffreys eavesdrops as they go to Harmer, who quickly realizes that someone tampered with the key to the medicine cabinet on the night of the murder. As she passes Isabel's room, Mary, hearing Isabel protest that Jeffreys is oversedating her, confronts her. Isabel weakly reveals that she was Jason's lover and that the older man was going to help her escape her husband when Jeffreys discovered the affair and killed Jason, then pinned the murder on Valerie. Mary tries to drag Isabel to Melling, but Jeffreys intercepts them and tricks Mary into accompanying him to the bell tower. There, he tells Mary he would do anything to keep Isabel's love. Jeffreys then attacks Mary, who rings the bell, alerting Sidney. Sidney rushes to her rescue and the two men fight until Sidney overpowers the doctor. As Jeffreys is arrested, Valerie and Sidney kiss, and Mary explains to the Mother Superior that she knows now that God chose his servants to help mankind.