The High Priest Oroveso and members of a Druidic cult meet at their temple to prepare for the rites of winter solstice. They pray to their god, Irminsul, that Norma, High Priestess and Oroveso's daughter, will declare war on the Romans and end their occupation.
After the Druids leave, Pollione, Proconsul of the Roman forces, arrives. His Celtic aide, Flavio, urges him to go, but Pollione insists on staying to meet Adalgisa, a priestess whom he loves. Flavio is distressed to learn that Pollione plans to abandon Norma, who has secretly borne him two children during the long Roman occupation. He cites the risks of engendering Norma's rage. Pollione acknowledges this, recounting a terrifying dream in which Norma savagely avenges herself for his infidelity.
The Druids return to perform their ritual. Norma warns that their warlike posture is futile and commands them to wait 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 on the Romans should the god Irminsul appear to her but reserves the right to kill Pollione herself. Privately, however, she longs that his love for her be rekindled.
As everyone leaves, Adalgisa stays behind to impore the gods to eradicate her feelings for Pollione, at which point the Proconsul returns and declares his love. Fearing that his dream may be a prophecy, he entreats Adalgisa to accompany him to Rome, where he has been ordered to return the next day. Adalgisa agrees.
Norma reveals her ambivalent feelings toward her children as she waits for them to be brought to her by Clotilde, who cares for them in secret. Aware of Pollione's order to return to Rome and his aloofness toward her, she fears he will abandon them. Hearing Adalgisa approach, Clotilde hides the children. Adalgisa confesses that she has fallen in love and asks Norma to forgive her and release her from her vows. Norma is sympathetic as she remembers her own love. Just as she agrees, Pollione appears. It becomes clear to Norma that it is he who has stirred the young priestess to love, and she furiously condemns him. Adalgisa, shocked to learn of the relationship between Pollione and Norma, vows to remain faithful to the High Priestess.
Although certain that her children are doomed to suffering as slaves or outcasts, Norma finds herself unable to kill them. Instead, she makes Adalgisa promise to go to Rome with Pollione and raise the children as her own. But Adalgisa insists on confronting Pollione and convincing him to return to Norma.
Oroveso and the Druids are gathered at the temple to secretly plan an attack on the Romans, when a Roman detachment arrives to inform Oroveso of the identity of the Roman Consul who will be replacing Pollione, a man reputed to be more savage and brutal. Oroveso takes this as a sign to abandon their planned attack and counsels his fellow Druids to feign submission and patiently await their chance to rebel.
Norma anxiously awaits the outcome of Adalgisa's mission and upon learning that it has failed, sounds the signal for war. As she prepares a sacrificial victim, uproar is heard in the inner temple and Pollione, who has been captured, is brought before her. Dismissing the Druids, she attempts to bargain with Pollione. He refuses to renounce Adalgisa, offering his own life instead.
Norma summons her followers and informs them that a priestess has broken her vow and must be sacrificed. However, it is not Adalgisa she indicts but herself. She reveals to Oroveso the existence of her children and exacts his promise to care for them. Pollione, moved, feels his love for her reborn and requests to die with her. Devastated, the Druids witness Norma's ascent to the pyre and, with her, the destruction of their temple.