The inhabitants of a small Swiss village are celebrating the forthcoming marriage of the orphan Amina and the wealthy farmer Elvino. The feudal lord Rodolfo, who is returning home after the death of his father, arrives to stay at the inn, where he flirts with Lisa, the hostess.
Suddenly a window openv , through which enters Amina. Lisa, jealous of Amina and her engagement to Elvino, hurries off to tell him of his fiancée's seeming assignation with Rodolfo. Rodolfo realizes that Amina is sleepwalking and, moved by her somnambulistic declaration of love, he leaves her on his couch.
Lisa and Elvino arrive to denounce Amina as an adulteress. Elvino calls off the wedding.
The village rallies round the distraught Amina and marches towards the castle to
beg Rodolfo for verification of Amina's innocence.
Amina is met on the way by Elvino, who rejects her and takes back his ring. He has decided to marry Lisa, who publicly rejoices in her good fortune. Rodolfo attempts to explain the truth, but Elvino is unconvinced by the tale of his fiancées somnambulism.
The crowd is then distracted by the sight of Amina walking across the roof of the mill. When
she reaches the ground, she is heard praying for the return of Elvino's love.
Moved by her devotion, Elvino replaces the ring on her finger. She wakes to find Elvino kneeling at her feet. The crowd rejoices as the lovers head towards the church and their wedding.