Scene One: The Sacred Grove of the Druids
Oroveso, the Arch druid, instructs some of the other Druids to watch for the appearance of the new moon and to signal its appearance with the sacred gong. When they ask if Norma will come to cut the sacred mistletoe, he answers in the affirmative, and they beg the god, Irminsul, inspire her to break the peace with the oppressive Romans. Oroveso assures them the god will free the land from the hated conquerors with their eagle standards.
After they leave, the Roman proconsul, Pollione appears with the centurion Flavio. He confides that, even though she has born him two children, he no longer loves Norma; he now loves the temple priestess, Adalgisa. He has dreamt of being with her in Rome when suddenly a great Druid mantel fell on her and she disappeared (Cavatina: "Meco all'altar di Venere" — With me at the altar of Venus). It was the vengeance of Norma. The gong sounds, and as the Druids approach, Pollione wows to destroy the Druid god (Caballetta: "Me protegge" — Protect me). The Romans leave.
The chorus announces the arrival of Norma, wreathed in verbena and bearing a golden sickle. They anticipate the destruction of the Romans, but she chides them, saying the Romans will not fall through human efforts. Time is not ripe for revenge; Roman spears are stronger than Celtic axes. When the people ask her for a prophesy, she announces that Rome will fall because of its own vices. As the moon appears, Norma extends her arms to heaven, and all prostrate themselves. She cuts the mistletoe and, joined by a chorus of priestesses, sings an invocation to the Chaste Goddess of the Moon (Casta diva). At the end of the rites she promises that, when the god demands Roman blood, she will let them know. She then says a private prayer for the return of Pollione's love.
All depart leaving Adalgisa alone. She begs the god to protect her even though she has betrayed her vows ("Sgombra e la sacra selva" -- The sacred wood is empty). When Pollione appears she begs him to leave her alone so she can return to her temple duties. He then tells her he is leaving for Rome and begs her to go with him; their love is more sacred than her heathen rites ("Vieni in Rome" — Come to Rome). She finally agrees.
Scene Two: Norma's House
Norma tells her companion, Clotilde, to hide her children. She both hates and loves them and wishes she could forget them. A hesitant Adalgisa arrives and begs Norma to help her; she has just promised to leave the temple and her fatherland. She tells of her meeting a man in the sacred wood and their subsequent secret trysts ("Sola, furtiva, al tempio" — Secretly, alone, at the temple). Norma knows just how she felt; she had the same feelings and experiences with Pollione, and she is ready to forgive the young priestess. After all, Adalgiia has not taken her final vows ("Ah, si, fa' core e abbracciami" -- Ah, yes, take heart and embrace me). When Norma asks which of the Gauls Adalgisa loves, she is told it is not a Gaul but a Roman. Just then Pollione appears; he is the one the young priestess loves! Norma curses the faithless man, for victimizing another woman, and orders him to leave. He curses the day he met Norma; and Adalgisa vows she will die in order to restore Pollione to Norma and their children. Just then the sacred gong sounds and the people summon Norma to the altar. Irminsul has called.
Scene One: Norma's House
Norma watches over her sleeping children and resolves to kill them so that they may be spared from the anguish they will face either in Gaul or in Rome. She cannot go through with it ("Tenere figli" — Tender sons). She sends for Adalgisa and asks her to promise to do something for her. Adalgisa agrees, and Norma tells her she has resolved to kill herself. She asks her to look after the children and take them to Rome with her, only promising not to let them become slaves ("Deh! Con te li prendi" — Ah! Take them with you). In spite of her earlier promise, Adalgisa refuses; she will never leave Gaul. Norma must keep her children. She herself, will go to the Roman camp and persuade Pollione to return to Norma ("Mira o Norma" — See, Oh Norma). They embrace and vow to remain friends forever ("Si, fino all'ore extreme" — Yes, until the last hour).
Scene Two: A Wooded area by temple
The Gallic warriors have gathered, determined to rebel; they are only waiting for Pollione to leave. Oroveso tells them the Roman will only be replaced by someone more cruel. He would like to fight, but they must pretend to be docile. The warriors reluctantly promise to obey for the time being.
Norma is sure Adalgisa will be able to persuade Pollione to return to her. When Clotilde tells her the young priestess's plea has failed, and the Roman has vowed to drag her off, from the altar if necessary, a furious Norma strikes the gong and declares war against the Romans. All cry out for war, blood and revenge ("Guerra! guerra! " — War! War!)
Clotilde then brings news that a Roman has been captured in the sacred cloister of the virgins. When Pollione is brought in, Oroveso tries to question him, but the Roman refuses to answer. Norma tries to strike him with her holy dagger but cannot. She then begs to speak to him alone. She will save him if only he will release Adalgisa ("In mia man alfin tu sei" — At last you are in my hands). He refuses, and she tells him how she considered killing their children. When he asks for the dagger so that he can kill himself, she responds that hundreds of the Romans will die and Adalgisa will perish in the flames. In spite of his pleas for her to kill him, she is resolved to punish him by sacrificing Adalgisa.
All return and Norma announces that a priestess had violated her vows. She orders a funeral pyre to be built. The Gauls call for the traitor's name, and Pollione begs her to show mercy. To the astonishment of all, Norma announces that she herself is the guilty one. Pollione finds his love returning in admiration for her bravery. She thinks of her children and asks Oroveso to care for them. When he refuses she begs him, Pollione adds his plea, and finally, a tearful Oroveso agrees. The people curse Norma and, as she and Pollione mount the pyre together, call for her death.