Among their adversaries is the proconsul, Pollione:  the secret lover of the priestess, Norma - who has given him two children - Pollione has recently fallen in love with the novice, Adalgisa and fears Norma's vengeance above all.  Norma appears and addresses an ecstatic prayer to the moon.

An assembly of druids prepares to celebrate the ritual harvest of sacred mistletoe.  They are led by Oroveso, Norma's father.  The druids invoke Irminsul and wait for a sign from God authorising them to break the truce concluded with the Romans so they can resume their war.

Norma learns of Pollione's betrayal with the same dread with which Adalgisa discovered her lover's lies - and she had been ready to sacrifice her vows for him!

Adalgisa feels guilty for violating her vow of chastity.  Whey she sees Pollione, she rejects him and begs him to leave.  Feeling hurt, Pollione attemps to convince her to follow him to Rome; Adalgisa at first resists but ends up agreeing to flee with hime the next day.

Born of an eternally damned union, Norma's children are now her shame:  the druidess first thinks of killing them as they sleep, but her maternal instincts hold her back.  She summon Adalgisa through her nannie, Clotilde.  and instructs her to take her children to Pollione, with whom she wishes to see her united.  But Adalgisa refuses:  not only has she renounced the Roman, but she is certain that he will in the end come backk to Norma.

Norma summons everyone and announces before her people that a false priestess has violated her sacred vows and, as expiationfor her sins, demands to be burned alive.  It is she herself who she is referring to.  In a supreme gesture, Norma steps atop the pyre and offers herself to the flames, accompanied by Pollione, moved by grace.

As war threatens once again, Norma's last illusion is shattered when she learns that Pollione intends to carry Adalgisa off to Rome.  She calls together priests and warriors and then launches attacks against the Romans.  With Pollione captured, Norma grabs a dagger, threatens him with it, and finally manages to convince him to sacrifice himself in order to save Adalgisa from being burned at the stake.