The opera takes place in a Swiss village early in the 19th century. ACT ONE opens in the village square where there's an inn, and the local mill, nestled in the background. A chorus of villagers is joyous, but the young woman Lisa, who owns the inn, is down in the dumps. Her former lover, Elvino, a successful farmer, is about to be engaged to someone else -- namely Amina, an orphan, brought up by Teresa, who runs the mill.
A man called Alessio rushes in and hugs an unenthusiastic Lisa. He's in love with her, but she pays him little attention. As the villagers arrive to celebrate Amina's engagement, she comes out of the mill with Teresa, and thanks them. Amina naively wishes Alessio and Lisa well, and Teresa notices Lisa's unfriendly reaction.
A notary enters, announcing the arrival of the groom, Elvino. With the notary as a witness, Elvino pledges everything he has to Amina, and she replies that all she has to offer in return is her heart. In their duet, Elvino gives Amina a ring that belonged to his mother, and a bouquet of wildflowers. They are now officially engaged.
The sound of coach wheels is heard, and a stranger enters; he's on his way to a nearby castle, but doesn't know exactly where it is. Lisa says he'll never make it by nightfall, and invites him to stay over at her inn. The villagers don't recognize him, but he remembers the mill and the countryside. He says that when he was young, he briefly lived in the castle he's looking for, and wonders what became of the count who owned it. The others tell him the count died long ago, and his heirs are missing. This stranger, Rodolfo, is quickly smitten by Amina, who reminds him of a youthful love.
It's now dusk and Teresa urges everyone to leave. She warns Rodolfo about a phantom, in white clothes, that haunts the area at night. Rodolfo is skeptical, but the villagers back up her story, describing the ghost to Rodolfo. He says a fervent good night to Amina, much to Elvino's indignation. Left alone, Elvino and Amina quarrel, and then make up, in a florid duet.
In the next scene, at the inn, Rodolfo is flirting with Lisa, who tells him that the local mayor has recognized him as the old count's legal heir, and thus the new lord of the castle. They're startled by a noise, and as Lisa hurriedly leaves, she drops her kerchief. Amina, dressed in white, comes in through the window; she's walking in her sleep, and Rodolfo realizes that she must be the "phantom" the villagers have been seeing.
In disjointed phrases Amina talks about her forthcoming marriage, Elvino's jealousy, and their quarrel. Rodolfo refrains from taking advantage of "this pure and innocent flower," and exits through the window, leaving Amina asleep in his room.
When the villagers arrive to welcome Rodolfo as castle's new lord, they're amused when they discover a girl in Rodolfo's bed. They're about to leave discreetly when Lisa walks in with Elvino and Teresa. Lisa triumphantly points to the sleeping girl, and everyone is shocked when they recognize Amina.
The commotion wakes Amina. She's confused, and says -- honestly -- that she has no idea how she wound up in Rodolfo's room. But the villagers denounce her, saying she's lying. Teresa, the only one who believes her, picks up the kerchief Lisa dropped earlier, mistaking it for Amina's, and puts it round Amina's neck. As the act ends, Elvino is convinced that Amina has betrayed him, and angrily calls off their wedding.
ACT TWO opens in a valley between the village and the local castle. The villagers are headed for the castle to put Amina's case to Count Rodolfo, while Amina seeks consolation from Teresa.
Meanwhile, Elvino is miserable, and treats Amina with anger. The villagers return, announcing that the Count has exonerated Amina. But that's not good enough for Elvino, who furiously snatches his ring from Amina, refusing to take her back.
In the next scene, in the village square, Lisa again brushes off Alessio's advances. With Amina in disgrace, she's now free to marry Elvino, who seems agreeable. He kisses her hand and leads her towards the church, while Rodolfo, arriving with the villagers, proclaims Amina's innocence.
In a quartet, the Count tries to explain to Elvino that Amina never betrayed him -- she really was sleep-walking. Meanwhile, Teresa is shocked to see that Elvino is about to marry Lisa, who helpfully points out that she was not the one who turned up in another man's room. But there's proof that she had been in Rodolfo's room! She left in a hurry when Amina turned up, dropping her kerchief in the process. And when Teresa produces that kerchief, Elvino realizes that Lisa has been lying.
Then, as everyone is in an uproar, a white figure appears on the roof of the mill. It's Amina, sleepwalking again. To the relief of the crowd, she descends without falling.
Still asleep, Amina begins to sing about Elvino and her grief over losing him. The beauty of her sentiment convinces everyone, including Elvino, that she is innocent after all. Elvino returns the engagement ring to Amina's finger, and she's gently awakened. She and Elvino are both overjoyed, and as the opera ends, the villagers hurry them off to the church to be married.
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