American gambler Nick Cain arrives in San Paolo, Italy for a vacation and is bemused by the lavish welcome he receives from hotel manager Miles Travers, casino owner Francisco Sperazza and police chief Armando Ceralde. Sperazza explains that a gambler of Nick's skill and fame draws crowds and that his presence at the San Paolo Casino will be of great benefit. In the evening, Nick goes to the casino and sees attractive American Kay Wonderly, whom he had noticed earlier at the hotel. Although Nick plays superbly, Kay loses all of her money and tells Sperazza that she cannot pay back the credit she obtained from the casino. Sperazza offers to forgive her debts, however, if she will keep Nick "happy," and interested in staying in San Paolo. Leery of the offer yet unable to refuse, Kay agrees, and Sperazza introduces her to Nick. The gambler is charmed by Kay, and after brushing off an American named Jack Kennedy who is desperate to talk to him, Nick spends a romantic evening with Kay. Upon returning to Nick's room for a nightcap, however, they are rendered unconscious by drugged liquor planted by Travers. When Nick awakens, he hears a terrified Kay protesting his innocence to Ceralde and Travers. As Sperazza enters, Ceralde explains to Nick that Kennedy, a U.S. Treasury agent, was killed in his room, and that the murder weapon was found in Nick's hand. Realizing that he is being framed, Nick grabs a pistol and forces the men to drink the drugged liquor. Nick escapes with Kay, then tries to steal a florist's truck, but the owner, Massine, pulls a gun on them and takes them to his apartment. There, Massine assures them that he means no harm and suggests that they hide in the nearby ruins of Alta Loma while Nick investigates Kennedy's murder. The next day, Nick and Kay drive to the ruins, where she confesses to him that she was hired by Sperazza to be nice to him. Nick readily forgives Kay, who then tells him that she saw Kennedy at the casino with a rich woman named Claudette Ambling. When night falls, Nick sneaks back into town, and with Massine's help, locates Claudette. At her mansion, Nick learns that she is romantically involved with both Sperazza and German businessman Eric Mueller, who appears to be the organizer of a suspicious deal in which Sperazza and Ceralde are involved. After eluding the police again, Nick returns to the ruins, where a fretful Kay welcomes him with a kiss. Nick explains the situation to her and assures her that he will return again before noon. Back in town, Massine directs Nick to a cafe where Mueller and the others are meeting. Nick eavesdrops as Mueller chastises Sperazza for endangering their operation by framing Nick for killing Kennedy, who was about to expose them. Massine follows the men when they leave, while Nick asks Toni, an Italian street urchin whom he has befriended, to keep Kay company. As Toni tries unsuccessfully to convince Kay to stay at the ruins and not go into town, Nick goes to Massine's apartment, where he discovers a large pile of cash. The supposedly poor florist then reveals that he is an agent for the Italian government, and that he, Kennedy and others have been tracking Mueller for five years. Massine further explains that Mueller possesses the "Hitler plates," with which Hitler had intended to forge foreign currency during World War II. Nick informs him that he overheard Mueller outlining plans to move the equipment, and Massine deduces that the forgers' hideout is in the caverns near the seashore, to which he had followed them. The caverns lead to dungeons below the city's ancient prison, and Massine rallies his men for the raid. Before they leave, however, Toni arrives and tells Nick that Kay has been arrested and taken to the prison. While Mueller has his men pack up the counterfeiting equipment, Ceralde tortures Kay for information about Nick. Toni insists on accompanying Nick and Massine on their mission, and while Massine and his men chase the gang, Nick ascends the spiral staircase leading from the dungeon to the prison. After socking Ceralde, Nick frees Kay, and Nick then aids Massine during a shootout with the forgers. Nick chases Sperazza and kills him after a fierce fight, then joins Kay on the shore. An Italian agent informs them that the dying Travers, who was in the gang, told them Mueller had fled to Tunis, and that Massine has already left in pursuit of him. With his arm around Kay, Nick then tells Toni that he is going to America to live with them.