The action takes place in a mountaintop Druidic Temple during the Roman occupation of Gaul.
Led by the high priest, Oroveso, members of the Druidic cult meet at their temple to prepare for the rites of the winter solstice. They pray to the god Irminsul that Norma, high priestess of the Druids and daughter of Oroveso, declare war on the Romans and thus end their hated occupation.
After the Druids have left the temple, Pollione, the proconsul of the Roman forces, emerges from Norma’s quarters. His aide, Flavio, urges him to leave the temple but Pollione insists on remaining to meet Adalgisa, a young priestess with whom he has fallen in love. Flavio is distressed to learn that Pollione intends to abandon Norma, who has secretly borne him two children during the long Roman occupation. He cites the risks of engendering the potent rage of the high priestess. Pollione acknowledges this, recounting a recent dream in which Norma savagely avenges herself for his infidelity.
The Druids return to perform their rituals. Norma warns the assembly that their warlike posture toward the Romans is futile and instructs them to wait patiently until Rome falls through its own vices. A group of virgin priestesses enters, Adalgisa among them. Norma cuts the sacred mistletoe and makes a burnt offering to the moon goddess. She promises to declare war against the Romans should the god Irminsul appear to her, but reserves the right to kill their leader Pollione herself. Privately, however, she expresses her longing that his love for her be rekindled.
As everyone clears the temple, Adalgisa remains to implore the gods to eradicate all romantic feelings for Pollione from her being. The proconsul returns and declares his love for her. He entreats Adalgisa to abandon her cloistered life and come with him to Rome, where he has been ordered to return the next day.
As Norma waits for her children to be brought to her by Clotilde, who cares for them in secret, she reveals her ambivalent feelings toward them. Aware of Pollione’s orders to return to Rome and his recent aloofness toward her, she fears he will abandon them all. Hearing the approach of Adalgisa, Clotilde hides the children inside the temple. The young priestess confesses that she has fallen in love and asks the high priestess to forgive her and release her from her vows. Norma is sympathetic as she remembers her own feelings of love toward Pollione. Just as she agrees to release Adalgisa from her vows of chastity, Pollione appears. It becomes immediately clear to Norma that it is he who has stirred the young priestess to love and she furiously condemns him.
Although certain that her children are doomed to lifelong suffering as either slaves or outcasts, Norma finds herself unable to kill them. Instead she makes Adalgisa promise to return to Rome with Pollione and raise them as her own. But Adalgisa insists on going to the Roman camp to convince Pollione to return to Norma and his children.
Oroveso and the Druid men gather at the temple for morning prayer and to secretly plan an attack on the Romans. A Roman detachment arrives informing Oroveso of the identity of the new proconsul who will be replacing Pollione in command. Oroveso then relates to his followers the violent nature of the new proconsul and concludes that it must be a sign from their god to abandon their planned attack. He counsels his followers to feign submission and patiently await their chance to rebel.
Norma anxiously awaits the outcome of Adalgisa’s mission. On learning that it has failed, she sounds the signal for war. As she prepares for the sacrificial victim, an uproar is heard in the inner temple, and Pollione, who has been captured trying to abduct Adalgisa from the temple, is brought before her. Dismissing the Druids, she attempts to bargain with Pollione for his life; but he refuses to renounce Adalgisa, offering his own life instead.
Norma summons her followers and informs them that one of the priestesses has broken her vows and must be sacrificed. But it is not Adalgisa she indicts but herself. She reveals to Oroveso the existence of her two children by Pollione and exacts his promise to care for them. Pollione, deeply moved by her noble sacrifices, feels his love for the high priestess reborn and requests to die with her on the funeral pyre. Devastated, the Druidic cult witness Norma’s ascent to the funeral pyre and with her the destruction of their temple.