The druids, led by their chief Oroveso, pray to their god Irminsul to inspire Oroveso’s daughter Norma to signal a revolt against the occupying Roman forces.
Pollione, the Roman proconsul, confesses to his associate Flavio that his love for Norma has cooled, even though she has borne him two children. The new object of his desire is Adalgisa, a priestess at the temple of Irminsul. He fears Norma’s wrath when she learns of his betrayal, and describes his ominous dream about a future with Adalgisa.
The druids reassemble, eager for action against their oppressors. Norma is irritated by their impatience, telling them this is not the time for vengeance. Secretly, she acknowledges that, although she has the power to punish Pollione for his desertion of her, she would defend him with her life should he return to her.
Adalgisa and Pollione meet alone. Adalgisa says that she has resolved to leave Pollione and return to her temple. He says that he is leaving at dawn for Rome, and pressures her into joining him.
Norma is distraught, torn between love and hatred for Pollione and her children by him. She has learned that he has been recalled to Rome, but admits to her confidante Clotilde that she is unsure whether or not he will take her with him.
With great trepidation, Adalgisa confesses to Norma that she has fallen in love, thus betraying her sacred vows. Unaware at first of the identity of Adalgisa’s lover, Norma is sympathetic to her plight: the story that Adalgisa tells of the course of her love stirs fond memories in Norma of the early days of her own affair with Pollione. She grants Adalgisa forgiveness and releases her from her vows.
When Norma learns that it is Adalgisa Pollione she loves, Norma vents her anger – not on Adalgisa, but on Pollione. Adalgisa is horrified by Pollione’s perfidy, and, despite her anguish, she swears to bring Pollione back to Norma and their children. As Norma is called to the altar, Adalgisa warns Pollione that the voice of Irminsul has sounded his death knell. Pollione is defiant.
Norma, in her anguish and despair, contemplates killing her children. But overwhelmed by her love for them, she recoils from the act. Instead, she resolves to atone for her sins by taking her own life, cleansing the earth of her existence. She entrusts her children to Adalgisa’s care with the instruction to take them to the Roman camp and, for the sake of the children’s future, to accept Pollione as her husband. But Adalgisa attempts to convince her that all may still be well: she will go to the camp and persuade Pollione to return to Norma. Eventually, Norma relents and accepts Adalgisa’s proposal.
The druids bide their time, awaiting the departure of Pollione. Oroveso brings dispiriting news: the next proconsul will be even harsher than Pollione. In the absence of any word from Norma, the rebellion must be stayed.
Norma tries to convince herself that Pollione will come back to her, but Clotilde has news that shatters such illusions: Adalgisa’s mission has failed. At last, Norma rouses her people to war, declaring that the mighty Roman eagle will be brought low. All that is required now is a sacrificial victim. Clotilde reports that the temple has been desecrated by a Roman who has been captured in the sacred cloister of the virgins. It is Pollione.
Oroveso and the druids urge Norma to strike down Pollione, but she cannot bring herself to do it. Dismissing the others, she tells him that she will spare him his life if he swears to renounce Adalgisa. He refuses and tells Norma that he is ready to die. She tells him that not only will he die, but that all the Romans will be slaughtered – and Adalgisa too, who must perish in the fire for having violated her vows. Pollione begs Norma to have mercy on Adalgisa, whatever his own fate.
Norma calls back the druids and announces a new sacrificial victim: a priestess who has broken her holy vows, betraying her people and the gods. She instructs the priests to prepare the funeral pyre. The victim is herself.
Seeing her sacrifice, Pollione’s love for Norma is reignited. He announces that he will die with her, and asks her forgiveness. Oroveso takes his leave of his daughter, and Norma begs him to take pity on her children. Together, Norma and Pollione go to the pyre.