ACT 1
The Druid high priest Oroveso, Norma’s father, enters a sacred grove accompanied by Gallic warriors and Druids. He announces that Norma will arrive at the rising of the moon to cut the sacred mistletoe. The men pray that she will finally lead them to war against the Romans.
As the Druids disperse, Pollione, the Roman Proconsul, arrives with his aide, Flavio. Norma, in violation of her vows as priestess, has loved Pollione secretly and has borne him two children. Pollione reveals that he no longer loves Norma, but now adores Adalgisa, a novice in the temple. Hearing the Druids announce Norma's arrival, Flavio urges Pollione to flee.
Norma enters, but refuses to call for war. Rome will fall, she explains, but through its own vices and not through the vengeance of the Gauls. She cuts the mistletoe and sings a prayer for peace to the moon goddess. Norma assures herself that if Pollione were to love her again, she would protect him. To the crowd, Norma replies that she will sound the gong for war if the gods demand it.
As the crowd disperses, Adalgisa appears, reflecting upon her attraction to Pollione. He arrives, tells her that he has been ordered back to Rome, and begs her to come away with him. Despite her misgivings, Adalgisa agrees to flee with him.
Norma arrives at her secret dwelling in the forest, where Clotilde cares for Norma's children. Norma fears that since Pollione has been recalled to Rome, he may attempt to take the children from her. Clotilde and the children hide when they hear Adalgisa approaching.
Adalgisa enters and confesses that she is in love with a man. As Adalgisa describes her situation, Norma is struck by how similar it seems to what she herself experienced when she first met Pollione. She tells Adalgisa that she will free her from her novice's vows, which have not yet bound her as a priestess. Pollione arrives and Norma realizes he is Adalgisa's lover. Horrified, Adalgisa swears that she will never accept Pollione now.
ACT 2
Afraid that her children would be outcasts in Gaul or slaves in Rome, Norma has decided to kill them. Even as she prepares to strike, she realizes that she cannot. She calls for Adalgisa and tells her that she plans to kill herself and that she wants Adalgisa to take the children to Pollione. Adalgisa renounces her love for Pollione and offers to try to heal the breach between him and Norma. In the name of Norma's children, Adalgisa convinces Norma to give up her suicidal ideas. The two women swear eternal devotion.
In the sacred grove, the Gallic warriors and Druids are planning their revolt against Rome, but Oroveso tells them that the Proconsul who will replace Pollione is even more fierce. The frustrated men agree to pretend to be docile in order to wait for the right time to strike.
In the temple of Irminsul, Clotilde tells Norma that Pollione has sworn to abduct Adalgisa. Norma is enraged and sounds the sacred gong to give the signal for battle.
The crowd brings the captive Pollione to Norma for judgment. While they demand death, Norma hesitates. Once they are alone, she offers to save Pollione if he will leave Gaul without Adalgisa. Pollione refuses and Norma threatens to kill their children. Pollione begs her to spare the innocent and offers to kill himself. Norma next threatens to attack the Romans and to burn Adalgisa, realizing that this is the surest way to hurt Pollione.
Norma calls everyone in and announces that a priestess must die for betraying her vows. The crowd insists on hearing the name of the traitor. As Pollione begs her to spare Adalgisa, Norma, realizing that she cannot condemn Adalgisa for the very crime she herself committed, confesses: "It is I!"
As she prepares to die, Norma asks her father to care for her children. Oroveso, stunned, refuses at first but eventually agrees. Moved by her courage, Pollione recognizes his re-born love for Norma and asks her pardon. The crowd demands vengeance for the betrayal of Norma's vows.