The short story Rain relates the events transpiring within a two-week period between a small group of English and American travelers in the South Pacific island of Tutuila. The primary plot details how one of the characters, an arrogant and self-righteous Christian missionary, attempts to reform another character, whom he mistakenly assumes is a common prostitute. The missionary uses his considerable political influence to control the situation and eventually spends several days alone with the alleged prostitute, cleansing her soul of sin. In the end, the woman seduces the missionary, who subsequently commits suicide rather than facing a suddenly uncertain future.

Dr. and Mrs. Macphail are traveling to Upolu by ship. They are accompanied on the long voyage by Mr. and Mrs. Davidson, missionaries returning to their designated area which includes some islands in the South Pacific. The ship is unexpectedly detained at Tutuila for a fortnight due to a case of infectious disease. The passengers find lodging in the expansive house of a local trader named Horn. At the house they pass the time in conversation and other so-called proper activities while other residents pursue smoking, gambling, and dancing.

One of the other residents, Miss Sadie Thompson, is a younger woman with unrefined manners and over-stylish dress. Thompson is traveling to a distant island where she has secured a job as a cashier. Davidson is particularly troubled by Thompson's lack of what he considers decent and moral behavior and in an apparent epiphany he decides that Thompson is a prostitute. Although Davidson is probably incorrect, Thompson's actions do not convince him that she is not a prostitute. Davidson attempts to have Thompson turned out of the house but there is no other lodging available. He then seeks to enforce his own standards of behavior on Thompson by pressuring various island inhabitants. Davidson is largely successful in cowing Thompson who, who determines to pass the time as quietly as possible to avoid further angering the politically powerful missionary.

Davidson is not satisfied, however, and convinces the island's governor to deport Thompson on the next ship leaving the island - b. By accident, it is bound for San Francisco. Thompson then seeks to have Davidson allow her to leave the island destined for any location other than San Francisco. Davidson, in an apparent second epiphany, decides that Thompson must be facing a prison sentence in San Francisco. Thompson seems to confirm Davidson's belief and states that she faces a three-year prison term if she is returned to San Francisco. To Dr. Macphail's surprise, Davidson is firm in his resolve to have Thompson deported to San Francisco.

Realizing that Davidson has mastered the situation and her fate, Thompson becomes despondent and then seeks forgiveness by repenting of her life of putative sin. She begins a prolonged ordeal of weeping, praying, and reading the Bible, e - assisted all the time by Davidson. For four days they are shut up together in Thompson's room while she grovels in her pajamas and begs him to counsel her.

On the morning of Thompson's scheduled departure Davidson is found dead on the beach. He has used a razor to slit his own throat. After a difficult few hours the Macphails and Mrs. Davidson return from the mortuary to find Thompson playing loud music, chatting up a sailor, and once again dressed in all her gaudy style. The penitent woman has evaporated with Davidson's suicide and Thompson is once again in her original guise. As a shocked Mrs. Davidson rushes to her room Thompson laughs and spits at her. When Dr. Macphail remonstrates with Thompson, she confronts him and tells him that all men are filthy pigs. In perhaps the only real epiphany of the narrative, Dr. Macphail realizes that Thompson's deliberately crafted behavior has led to the seduction and subsequent suicide of the stern Davidson.