Alessio, a peasant lad, is also in love with the landlady.  Such is the state of affairs on the day before the wedding.  Rodolfo, the young lord of the village, next appears upon the scene.  He has arrived incognito for the purpose of looking up his estates, and stops at Lisa’s inn, where he meets Amina.  He gives her many pretty compliments, much to the dissatisfaction of the half-jealous Elvino, who is inclined to quarrel with the disturber of his peace of mind.  Amina, who is subject to fits of somnambulism, has been mistaken for a ghost by the peasants, and they warn Rodolfo that the village is haunted.  The information, however, does not disturb him, and he quietly retires to his chamber.  The officious Lisa also enters, and a playful scene of flirtation ensues, during which Amina enters the room, walking in her sleep.  Lisa seeks shelter in a closet.  Rodolfo, to escape from the embarrassment of the situation, leaves the apartment, and Amina reclines upon the bed as if it were her own.  The malicious Lisa hurries from the room to inform Elvino of what she has seen, and thoughtlessly leaves her handkerchief.  Elvino rushes to the spot with other villagers, and finding Amina, as Lisa had described, declares that she is guilty, and leaves her.  Awakened by the noise, the unfortunate girl, realizing the situation, sorrowfully throws herself into Teresa’s arms.  The villagers implore Rodolfo to acquit Amina of any blame, and he stoutly protests her innocence; but it is of no avail in satisfying Elvino, who straightway offers his hand to Lisa.  In the last act Amina is seen stepping from the window of the mill in her sleep.  She crosses a frail bridge which yields beneath her weight and threatens to precipitate her upon the wheel below; but she passes it in safety, descends to the ground, and walks into her lover’s arms amid the jubilant songs of the villagers.  Elvino is convinced of her innocence, and they are wedded at once, while the discovery of Lisa’s handkerchief in Rodolfo’s room pronounces her the faithless one.

Such is the simple little pastoral story to which Bellini has set some of his most beautiful melodies, the most striking of which are the aria, “Sovra il sen,” in the third scene of the first act, where Amina declares her happiness to Teresa; the beautiful aria for barytone in the sixth scene, “Vi ravviso,” descriptive of Rodolfo’s delight in revisiting the scenes of his youth; the playful duet between Amina and Elvino, “Mai piu dubbi!” in which she rebukes him for his jealousy; the humorous and very characteristic chorus of the villagers in the tenth scene, “Osservate, l’uscio e aperto,” as they tiptoe into Rodolfo’s apartment; the duet, “O mio dolor,” in the next scene, in which Amina asserts her innocence; the aria for tenor in the third scene of the second act, “Tutto e sciolto,” in which Elvino bemoans his sad lot; and that joyous ecstatic outburst of birdlike melody, “Ah! non giunge,” which closes the opera.  In fact, “Sonnambula” is so replete with melodies of the purest and tenderest kind, that it is difficult to specify particular ones.  It is exquisitely idyllic throughout, and the music is as quiet, peaceful, simple, and tender as the charming pastoral scenes it illustrates.