Time:1849 to 1850.
Place: A mining camp at the foot of the Cloudy Mountains, California.
Act 1
Inside the Polka Saloon

A group of Gold Rush miners enter the "Polka" saloon after a day working at the mine ("Hello! Hello! Alla 'Polka'"). After a song by traveling minstrel Jake Wallace ("Che faranno i vecchi miei"), one of the miners, Jim Larkens, is homesick and the miners collect enough money for his fare home ("Jim, perchè piangi?").

A group of miners playing cards discover that Sid is cheating and want to attack him. Sheriff Jack Rance quiets the fight and pins two cards to Sid's jacket, as a sign of a cheat.

A Wells Fargo agent, Ashby, enters and announces that he is chasing the bandit Ramerrez and his gang of Mexicans. Rance toasts Minnie, the woman who owns the saloon, as his future wife, which makes Sonora jealous. The two men begin to fight. Rance draws his revolver but at that moment, a shot rings out and Minnie stands next to the bar with a rifle in her hands ("Hello, Minnie!"). She gives the miners a reading lesson from the Bible ("Dove eravamo?").

The Pony Express rider arrives ("La posta!") and delivers a telegram from Nina Micheltorena, offering to reveal Ramerrez's hideout. The sheriff tells Minnie that he loves her, but Minnie puts him off as she is waiting for the right man ("Ti voglio bene, Minnie").

A stranger enters the saloon and asks for a whisky and water. He introduces himself as Dick Johnson from Sacramento, whom Minnie had met earlier. Johnson invites Minnie to dance with him and she accepts. Angrily, Rance watches them.

Ashby returns with the captured Ramerrez gang member, Castro. Upon seeing his leader, Johnson, in the saloon, Castro agrees to lead Rance, Ashby and the miners in a search for Ramerrez, and the group then follows him on a false trail and in what turns out to be a wild goose chase. But before Castro leaves, he whispers to Johnson that somebody will whistle and Johnson must reply to confirm that the place is clear. A whistle is heard, but Johnson fails to reply.

Minnie shows Johnson the keg of gold that she and the miners take turns to guard at night and Johnson reassures her that the gold will be safe there. Before he leaves the saloon, he promises to visit her at her cabin. They confess their love for each other. Minnie begins to cry, and Johnson comforts her before he leaves.

Act 2
Minnie's dwelling, later that evening

Wowkle, a Native American woman who is Minnie's servant, her lover Billy Jackrabbit and their baby are present as Minnie enters, wanting to get ready for Johnson's visit. Johnson enters Minnie's cabin and she tells him all about her life. It begins to snow. They kiss and Minnie asks him to stay till morning. He denies knowing Nina Micheltorena. As Johnson hides, a posse enters looking for Ramerrez and reveal to Minnie that Johnson is the bandit Ramerrez himself. Angry, she orders Johnson to leave. After he leaves, Minnie hears a gunshot and she knows Johnson has been shot. Johnson staggers in and collapses, Minnie helps him by hiding him up in the loft. Rance enters Minnie's cabin looking for the bandit and is about to give up searching for Johnson when drops of blood fall on his hand. Rance forces Johnson to climb down. Minnie desperately makes Rance an offer: if she beats him at poker, he must let Johnson go free; if Rance wins, she will marry him. Hiding some cards in her stockings, Minnie cheats and wins. Rance honors the deal and Minnie throws herself on the unconscious Johnson on the floor.

Act 3
In the Great Californian Forest at dawn, sometime later

Johnson is again on the run from Ashby and the miners. Nick and Rance are discussing Johnson and wonder what Minnie sees in him when Ashby arrives in triumph: Johnson has been captured. Rance and the miners all want Johnson to be hanged. Johnson accepts the sentence and only asks the miners not to tell Minnie about his capture and his fate ("Ch'ella mi creda"). Minnie arrives, armed with a pistol, just before the execution and throws herself in front of Johnson to protect him. While Rance tries to proceed, she convinces the miners that they owe her too much to kill the man she loves, and asks them to forgive him ("Ah! Ah! E Minnie!"). One by one, the miners yield to her plea ("E anche tu lo vorrai, Joe"). Rance is not happy but finally he too gives in. Sonora unties Johnson and sets him free. The miners bid Minnie farewell ("Le tue parole sono di Dio"). Minnie and Johnson leave California to start a new life together.