Act I

The peasants are singing to celebrate the marriage of the rich landowner Elvino to Amina, an orphan adopted by Teresa. In the general rejoicing Lisa cannot find peace: she complains because of her unrequited love for Elvino. In the meantime she ignores the love which Alessio cherishes towards her. While awaiting the bridegroom, Amina happily answers her village friends. Elvino finally arrives after the notary and offers the wedding ring to Amina. The idyll is broken by the unexpected arrival of a carriage bearing Count Rodolfo, the son of the late Lord of the village, who, after many years absence, is not recognized and prefers to remain incognito. He takes up lodgings in Lisa’s inn and pays compliments to the young bride, thus arousing Elvino’s jealousy.

In his room in the inn, Count Rodolfo pays court to Lisa who seems quite willing, but runs out, dropping a handkerchief, when a sound is heard outside; at that moment Amina arrives, asleep, in her nightdress, repeating the name of Elvino and describing the vision she has of the coming wedding ceremony; then she lies clown on the couch. Rodolfo remains disconcerted and uncertain as to what to do, then leaves the room. Right in this predicament the crowd of villagers enters to congratulate the Count, whose identity they have discovered. In this way everyone sees Amina, asleep in Rodolfo’s room. On waking, the young girl tries to justify herself and pleads her own innocence but nobody believes her. Elvino, seized by jealousy, repudiates her.

Act II

A group of peasants go to see the Count to persuade him to defend Amina who, in the meantime, accompanied by Teresa, crosses the path of Elvino who takes back the ring.

In the village, Lisa, taking advantage of the new situation, is about to marry Eivino who has accepted the marriage despite the Count’s repeated assurances that Amina is innocent. The village is celebrating once again but when Lisa and Elvino pass in front of Teresa’s mill, the latter accuses Lisa of having committed the same act as Amina, declaring that she has found a handkerchief of hers in Rodolfo’s room; Elvino is angry and jealous and rejects this marriage too. Suddenly the sleeping Amina appears on the edge of the roof, thus confirming the words spoken by the Count in her defence. Amina, still sleepwalking, comes down into the street singing her love for Elvino; the latter, repentant and with a change of heart, takes her in his arms, where she awakens. The festivities start once again and preparations are finally made for the wedding.