The action takes place in rural Switzerland
Act I
Scene 1. An alpine village, close to the mill
The villagers are revelling in the preparations for the wedding of Elvino and Amina, Teresa the miller’s foster daughter. The chorus is led by the merry-maker Alessio. But it is only Lisa, the mistress of the inn, who is not enjoying the festivities. She herself is in love with Elvino and, regardless of her liaison with Alessio, she would marry Elvino in a trice. Teresa brings Amina out. Alessio and the villagers congratulate her. The notary arrives to conclude the contract, and he is eventually followed by Elvino, the groom himself.
Suddenly a stranger appears. He is going to the castle, but because of the poor road and the fact that night is falling he must seek shelter in the village. Lisa and Teresa explain that he has caught them in the middle of a wedding. The stranger (who is, in fact, Count Rodolfo) joins the villagers in their celebrations and pays the bride some compliments. Teresa says that there is a ghost in the village who walks as darkness falls. The frightened villagers depart. Lisa takes Rodolfo to the inn. Amina and Elvino remain on the road. Elvino rebukes his fiancée for accepting the stranger’s attentions. Amina pacifies him. Her explanation concludes with assurances of eternal love and fidelity.
Scene 2. Rodolfo’s room in the inn
Lisa informs Rodolfo that his disguise has been exposed and the villagers are now coming to welcome their seigneur. Lisa departs, dropping a handkerchief which Rodolfo picks up and places on the headboard of the bed. Suddenly the window opens and a woman in a white dress appears. It is Amina. She is behaving strangely. Rodolfo understands that he is faced with a sleepwalker, wandering and talking in her sleep. Now he understands the villagers’ fears and the rumours of a ghost. Lisa overhears Amina speaking with Rodolfo and decides to call Elvino. Meanwhile, Amina lies down on Rodolfo’s bed and falls asleep. Rodolfo, deciding not to disturb the sleeping girl, extinguishes the light and leaves the room. Headed by Alessio, the villagers enter the room, but on seeing a woman on the bed they want to leave. Suddenly Lisa runs in with Elvino. The room is lit up and she mercilessly rouses Amina. The latter is unable to understand where she is. Elvino accuses Amina of infidelity. None of the other villagers believe her either. Amina loses consciousness. Only Teresa remains with her.
Act II
Scene 1. The road from the village to the castle
The villagers resolve to approach the Count in order to discover whether Amina is guilty of infidelity or not. Amina and Teresa go with them. They meet Elvino on the way. He doesn’t want to hear anyone’s explanations, removes the ring from Amina’s finger and flees.
Scene 2. Close to the mill
Lisa declares to Alessio that she is marrying Elvino. Elvino appears. He has decided to marry Lisa out of despair. Rodolfo enters. The Count tries to bring Elvino to reason, but the latter doesn’t understand what the Count is saying, nor does he have any wish to. Teresa comes out of the miller’s house and asks them to be quiet as the miserable Amina has just fallen asleep. Teresa gives Lisa the handkerchief the latter had left in the Count’s bedroom. Elvino is incensed: so Lisa, too, has been unfaithful to him with the Count. Rodolfo tries to explain. At this very moment Amina, dressed from head to toe in white, comes out of a window of the miller’s house. Sleepwalking, she crosses the flimsy bridge that leads across the mountain stream to the other side. Everyone is frozen in terror – one false step and the girl will die. Nevertheless, the sleepwalker successfully reaches the other side. She falls to her knees, prays and bemoans her fiancé for not believing her, even though she is chaste and innocent. Elvino approaches Amina and puts the ring back on her finger.
