Scene 1: A village, a mill in the background
Lisa, the proprietress of the inn, is consumed with jealousy as the betrothal procession of Amina and Elvino, who had once been betrothed to her, approaches. She spurns the lovelorn Alessio. Amina thanks her friends for their kind wishes and particularly her foster-mother Teresa, owner of the mill, who had adopted her as an orphan. She thanks Alessio, who had composed the wedding song and organised the celebrations, wishing him well in his courtship of Lisa, who continues to reject his advances. Elvino arrives, having stopped on his way at his mother's grave to ask her blessing on Amina. He gives Amina his mother's ring and they exchange vows.
A stranger arrives, asking the way to the castle. Lisa points out that it is getting late and he will not reach it before dark and offers him lodging at her inn. The newcomer, who surprises the villagers by his familiarity with the locality, asks about the celebrations and admires Amina, who reminds him of a girl he had loved long ago. He admits to having once stayed in the castle, whose lord has been dead for four years. When Teresa explains that his son had vanished some years previously, the stranger assures them that he is alive and will return.
As darkness approaches the villagers warn him that it is time to be indoors to avoid the village phantom, but he is not superstitious and assures them that they will soon be free of the apparition. Elvino is jealous of the stranger's admiration of Amina; he is jealous even of the breezes that caress her, but he promises her he will reform.
Scene 2: A room in the inn
Lisa tells the stranger that he has been recognised as Rodolfo, the long-lost son of the count, and warns him that the village is preparing a formal welcome. Meanwhile she will be the first to pay her respects. She is flattered when he begins a flirtation with her, but runs out, dropping a handkerchief, when a sound is heard outside.
It is Amina, who enters the room, walking in her sleep. Rodolfo, realising that her nocturnal wanderings have given rise to the story of the village phantom, is about to take advantage of her helpless state but is struck by her obvious innocence and refrains. She falls asleep on the sofa and he goes outside as the villagers are heard advancing on the inn to welcome their new lord. Lisa points to the sleeping Amina; and Elvino, believing her faithless, rejects her in fury. Only Teresa believes in her innocence.
Scene 1: A wood
On their way to ask the count to attest to Amina's innocence, the villagers meet Amina and Teresa, on a similar mission. Elvino continues to reject Amina, even when the count sends a message that she is innocent. Elvino is not convinced and takes back the ring, though he is unable to tear her image from his heart.
Scene 2: The village, as in Act I
Elvino has decided to marry Lisa. They are about to go to the church when Rodolfo tries to explain that Amina is innocent because she had not come to his room awake - she is a somnambulist, a sleepwalker, but Elvino refuses to believe him.
Teresa begs the villagers to be quiet, because Amina has at last fallen into an exhausted sleep. Learning of the impending marriage, she confronts Lisa, who says that she has never been found alone in a man's room. Teresa produces the handkerchief Lisa had dropped. The Count refuses to comment, but continues to assert Amina's virtue. Elvino demands proof, which is dramatically produced when Amina is seen walking in her sleep across the high, dangerously unstable mill bridge. Rodolfo warns that to wake her would be fatal, so all watch as she relives her betrothal and her grief at Elvino's rejection. When she reaches the other side safely, Elvino calls to her and she wakes to find herself in his arms, to the rejoicing of all.