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 cheater.
A Wells Fargo agent, Ashby, enters and announces that he is chasing the bandit Ramerrez and his gang of Mexicans. Rance toasts Minnie, the girl 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?").