Amidst the hurly-burly of Broadway, Sarah Brown and her gallant band from the Save-A-Soul Mission endeavour to recruit sinners. Nathan Detroit and the gambling fraternity are more concerned with finding a place for their illegal floating crap game and Adelaide, Nathan's long-time fiancée, is preoccupied with trying to get him to the altar.
Sky Masterson saunters into town and gets suckered into a bet with Nathan that he can take any doll he likes to Havana. Nathan names Sarah Brown, and, undaunted, Sky presents himself at the Mission as a sinner. When this doesn't work, he guarantees her a dozen sinners in return for having dinner with him - in Havana. Sarah's response is he's not her type of guy ("I'll Know").
At The Hot Box night club where Adelaide stars ("Bushel and a Peck"), Nathan promise; her he will reform, Meanwhile, his side-kicks Benny and Nicely-Nicely complain she's taking his mind off his work ("Guys and Dolls"). The arrival of General Cartwright, intent on closing the Mission through lack of support, galvanises Sarah into accepting Sky's invitation. Nathan, still hoping to win his bet and the thousand bucks he needs to set up the game, is having trouble with the gambling fraternity - Big Jule, Harry the Horse and Rusty Charlie, to name but a few - who are getting increasingly restless. The appearance among them of Lt. Brannigan spells trouble, but the day is saved when they tell him they're celebrating Nathan's and Adelaide's elopement.
In Havana, Sarah becomes decidedly more friendly after drinking several of the local 'milkshakes' and causes a riot at El Cafe Cubano when she objects to Sky dancing with another girl ("If I Were A Bell"). He is shamed into telling her about the bet and takes her back to New 'fork where they arrive just before dawn -sober, and in love ("I've Never Been In Love Before"). Their idyllic moment is soon shattered; as Arvide -Sarah's grandfather and the Mission Band return wearily from all-night canvassing, the sound of a police siren sends the gamblers scuttling out of the Mission, where Nathan had finally found a place for the game.
Adelaide entertains The Hot Box customers ("Take Back Your Mink"), still blissfully believing she is about to become Mrs Nathan Detroit. Arvide consoles Sarah as the deadline for closing the Mission approaches and tries to put in a good word for Sky, who is still determined to honour his pledge. He follows the gamblers - and the game - into the sewer and bets each of them a thousand dollars against their souls on the roll of the dice ("Luck Be A Lady"). When he wins, they are obliged to attend the meeting at the Mission where - after some initial reluctance - they join Nicely-Nicely in giving testimony ("Sit Down, You're Rocking the Boat"). Though Lt. Brannigan, hot on their heels, remains unconvinced, General Cartwright is impressed.
When Sarah learns that Sky told Nathan he had lost the bet and did not take her to Havana, she realises he can't be all bad. She and Adelaide commiserate with each other on the problems of getting their guys to change their ways and decide the answer is to marry them first, which they do, and live happily ever after.