Setting: A small village. The villagers are celebrating the upcoming wedding of Amina and Elvino. A stranger, Rodolfo, arrives and asks for directions to the nearby castle. Lisa soon convinces Rodolfo to spend the night at her inn. The villagers warn him about the mysterious “woman in white,” an apparition that haunts the village every night. Later that evening, in his room at the inn, Rodolfo flirts with Lisa, but their rendezvous is interrupted by the arrival of the phantom. It turns out to be Amina, who is subject to spells of sleep-walking.  Delighted by Amina’s arrival, Lisa runs off to find Elvino, since it gives her the opportunity to prove Amina’s infidelity. Amina falls asleep on a couch in Rodolfo’s room. Elvino, Lisa, and the villagers return to find her still asleep and Elvino, convinced of her treachery, calls off their engagement.

The villagers travel to Rodolfo’s castle, to secure his vindication of Amina’s honor. Amina pleads her innocence with Elvino, but he demands the return of the engagement ring. The villagers return, having secured Rodolfo’s pledge that Amina was faithful to Elvino, but he remains unmoved. Later, back in the village, Elvino announces that he is going to marry Lisa right away. Rodolfo arrives, insisting on Amina’s innocence and giving a lecture on the phenomenon of somnambulism, which none of the villagers believes. Lisa proclaims that, unlike Amina, she has never set foot in a man’s bedroom, but one of the villagers produces a scarf which she had left behind in Rodolfo’s room at the inn. Elvino despairs of ever finding a wife, just as Amina appears sleep-walking along a nearby rooftop. She sings with such conviction of her love for Elvino and her sorrow at losing him that Elvino is finally convinced of her innocence. Back on the ground, Amina is awakened by Elvino returning the wedding ring to her finger, and they are reunited.