Gaul, under Roman occupation, in the first century B.C. During the night in the sacred forest of the god Irminsul, Oroveso, the Druid high priest, and the Druids wait for the moon to rise. It is then that Norma, his daughter and high priestess, is expected to perform the sacred rite of cutting the mistletoe and invoking the Goddess of Moon to learn if the revolt against the Roman oppressor should be signaled. Unbeknownst to all, Norma has broken her vow of chastity by falling in love with the Roman proconsul Pollione and secretly giving him two sons. When Oroveso and the Druids disperse, Pollione and his lieutenant Flavio enter. Their presence in the forest is forbidden, but Pollione is searching for Adalgisa, a novice at the temple of Irminsul, and his new love. Pollione has bad premonitions that appeared to him in a dream, and fears that Norma will take a revenge on Adalgisa if she finds out the truth. The approach of the Druids causes the two Romans to retreat into the forest. Oroveso and the Druids call for vengeance against the Romans, but Norma hesitates. Fearing for Polliones safety, she declares that the time for an uprising has not come yet. Publicly, Norma proclaims the proconsul shall be punished, but privately she reveals her love and devotion to him. After the Druids leave, Adalgisa enters and is soon joined by Pollione. Having broken her oath as a priestess, just as Norma did before her, Adalgisa is troubled and has her doubts over this dangerous love. She is eventually persuaded by Pollione to follow him to Rome, and they agree to meet the following evening.
Inside her remote dwelling, Norma confides in Clotilde, who only knows about Normas relationship with Pollione and their children. Norma suffers both at the sight and the absence of her two children and fears that Pollione, who is recalled to Rome, will not take her with him. Adalgisa enters and confesses her transgression. As she describes her first clandestine meeting in forest, Norma recalls a very similar encounter with Pollione she herself had, and is ready to forgive. However, she soon realizes they have both been seduced by the same man. Pollione enters to bid farewell to Norma, and she immediately condemns his lack of fidelity. Pollione urges Adalgisa to leave with him, but she resists, preferring to die rather than allow Pollione to abandon Norma and their children.
To punish Pollione, Norma intends to murder her children, but is deeply moved by their innocence, and cannot bring herself to commit the dreadful act. She contemplates suicide instead, and after summoning Adalgisa, implores the young woman to marry Pollione and take care of the children after her death. Moved by Normas sacrifice, Adalgisa promises to go to Pollione and convince him to return to the mother of his offspring. It is Normas turn to be moved now, and the two women swear eternal friendship.
In the sacred forest, the Druids are still agitated and disturbed by the Roman presence. Oroveso has learned that a far more severe commander will be taking Polliones place. But according to Normas invocations, the time of the revolt has still not come.
Norma is full of hope that Pollione will ultimately return to her. However, Clotilde brings news that Adalgisa has been unsuccessful in her suit, and that Pollione intends to steal her away that evening. Normas fury comes to a boil; she incites the warriors to battle by striking the sacred gong three times. The Druids prepare for war. Meanwhile, Pollione has been discovered in the cloister of novices trying to abduct one virgin, and must be put to death as a result of the sacrilege. Norma intends to stab him, but then decides to interrogate him in private. If he will abandon all thoughts of Adalgisa, she will spare his life. When Pollione refuses, she threatens to kill their children and burn Adalgisa alive for breaking her vows. He asks only to die. Besides herself, Norma summons Oroveso and the Druids. She proclaims that a priestess has been unfaithful to her vows and must be ritually sacrificed at stake. Pollione begs her to be silent and to spare Adalgisa life. However, to everyones shock and utter surprise, Norma announces herself as the guilty priestess. She implores her father to take care of her children. As she is about to mount the pyre, Pollione, moved by her nobility and courage, feels his love for Norma reborn. The two reunited lovers proudly and serenely ascend into the flames.