A big-city gambler bets that he can seduce a Salvation Army girl.

Nathan Detroit, the financially strapped organizer of the oldest, established, permanent, floating crap game in New York, is trying to find a new venue, despite the scrutiny of police lieutenant Brannigan, who is determined to stop it once and for all. Nathan and his cohorts, Nicely-Nicely Johnson and Benny Southstreet, are equally determined to get the game up and running again. Because of the "heat" from Brannigan, Nathan's only option for the site is the Biltmore Garage, whose owner is demanding a $1,000 advance. Nathan, who is so broke that he cannot afford to buy an anniversary gift for Miss Adelaide, his fiancée of fourteen years, sees a way out of his predicament when he hears that Sky Masterson, a gambler noted for making large and unusual bets, is having lunch in nearby Mindy's restaurant. After Nathan fails to entice Sky to bet on whether or not Mindy's sold more cheesecake or strudel the day before, he gets an idea when Sky boasts that he could take any woman he wanted with him to Havana the next day. Seeing the Save-A-Soul Mission band march by, Nathan bets Sky $1,000 that he will not be able to take the mission's leader, Sergeant Sarah Brown. Although Sky is chagrined that he has fallen for a "sucker bet," he goes to the mission and announces to Sarah and her uncle, Arvid Abernathy, that he is a sinner who wants to reform. She is attracted to Sky, but suspicious of his motives and not impressed by his ability to quote the Bible. Learning that the mission is having trouble attracting sinners, he gives Sarah his marker to deliver twelve bona fide sinners at their midnight prayer meeting in two days if she will go to dinner with him the next night. Sarah declines, but Sky refuses to take back his marker and tells her that he will pick her up at noon--because his favorite restaurant is in Havana. She assures him that she is only interested in "upright squares" but momentarily responds when he kisses her. That night, at the Hot Box Club, Adelaide, the club's star performer, tells Nathan that she will be getting a raise next week and will finally earn enough money for them to get married. Adelaide pressures Nathan with the news that her mother thinks they have been married for years and have five children. Just then Laverne, one of the club's dancers, chastises Nathan for luring her boyfriend into his crap game. Adelaide realizes that he again has gone back on his promise to give up the game and screams at him to get out. After Nathan leaves, Adelaide, who has been suffering from a chronic cold, reads a book on psychology that describes her symptoms as psychosomatic reactions to her uncertainty about Nathan. The next day, General Cartwright, Sarah's supervisor, tells her that their organization will have to close the New York branch because it has not attracted any sinners. Just then Sky arrives at the mission and asks Cartwright to give Sarah thirty-six hours to prove that the mission is a success. With Arvid's encouragement, Sarah looks at Sky's marker, and tells Cartwight she can guarantee that at least twelve sinners will be at the midnight prayer meeting the next night. In Havana, Sarah's prim demeanor is overcome after Sky orders her several milk drinks liberally flavored with rum. At a nightclub, she and Sky talk about love and soon begin to kiss. At another club, when a Cuban woman flirts with Sky and takes him onto the dance floor, she and Sarah start a fight that turns into a huge brawl. Later, when Sarah is sober, she admits to having fun, and after he tells her about the bet, says she does not mind. They arrive back in New York just before dawn, and as they near the mission, they hear police sirens. Nicely-Nicely, who has been dozing across the street, rushes into the mission and alerts dozens of gamblers, who pour out of the mission's back room, narrowly evading Brannigan and the police. An angry Brannigan accuses Sarah of knowing about the game. Although Sarah is incorrect in assuming that Sky is involved, he refuses to deny it. The next night, Sky goes to the Hot Box and sees Nicely-Nicely, who laments that Nathan has asked him to tell Adelaide that the elopement they planned is off. Sky volunteers to tell Adelaide, who knows Nathan is not coming because of a crap game. Sky tries to be sympathetic to her but leaves when she admonishes that he will know how bad she feels when someday he falls in love with the wrong person. At the mission, Arvid tries to convince Sarah that Sky had nothing to do with the crap game, but she admits that she was more worried thinking that someday it would be Sky running from the police. Just then, Sky and Nicely-Nicely arrive at the mission and remind her of the marker. She tells Sky that they are now even and leaves, but Arvid, knowing that Sarah is in love with Sky, tells him that he must make good on the marker or everyone in town will know he is a welsher. Meanwhile, in the city sewer, the crap game has been going on for twenty-four hours. Nathan and his friends want to quit, but Big Jule, a notorious hoodlum from Detroit, intimidates them and insists on playing with his own dice until he wins back his $25,000 loss. When Nathan sees that the dice have no spots, Big Jule claims he "knows" where the spots are and wins back everything, even Nathan's commission. Now Sky arrives and, after punching Big Jule and taking his gun, promises to do one roll of the dice betting all of the money against everyone coming to the midnight prayer meeting. He wins, and the men all go toward the mission. On the way, Nathan run into Adelaide at Mindy's. He assures her he loves her, but when he says he cannot elope right now and leaves, she sneezes and sobs. At midnight, Sarah tells Cartwright that she has failed, just before a large number of "sinners" arrive with Sky. He curtly asks Sarah for his marker and leaves after asking Nathan to keep the markers from the others during the prayer meeting. As Cartwright asks for testimony about their sins, the men reluctantly talk about the crap game until Nicely-Nicely stands up and expresses true conversion. When Brannigan arrives at the mission, he tells Cartwright that there had been a crap game at the mission the night before, but Sarah lies by saying it never happened. Nathan then privately tells Sarah about the $1,000 bet with Sky. When Nathan relates that Sky paid the bet after saying that Sarah did not go to Havana, she runs after Sky. Soon Times Square is decorated for a double wedding as Adelaide marries Nathan and Sarah marries Sky.