Egypt, fourth century A.D. The old Palémon and his fellow monks await the return of Athanaël, one of their brothers. He enters and tells them how he found the city of Alexandria in a state of sin and under the influence of the courtesan Thaïs, whom he remembers seeing once in his youth before he joined the order. Athanaël believes her behavior is an affront to God, and despite Palémon’s warnings not to interfere with matters of the secular world he is determined to convert her to a life of devotion and faith. While the other monks retire to bed, Athanaël lies awake and has a vision of Thaïs. He prays for strength and, as dawn breaks, wakes his brothers, telling them he must journey back through the desert and save Thaïs’s soul. Palémon repeats his warning as Athanaël leaves for Alexandria.
Athanaël arrives in the city of his birth, whose luxury and learning he left behind to pursue a spiritual life (“Voilà donc la terrible cité”). He comes to the house of the wealthy Nicias, a friend of his youth, who tells him that Thaïs is his mistress of the moment, but that he will soon lose her since he has run out of money. Athanaël replies that he has come to convert Thaïs. Nicias laughs at this and warns him that Venus will take revenge for it, but he agrees to introduce Athanaël to her. He asks his slave girls to dress the monk in new clothes. Thaïs appears, saying goodbye to Nicias after a week of love. Athanaël confronts her and announces his intention, which she dismisses lightly, asking him why he denies his true nature and doesn’t give in to love. As she is about to take off her clothes, he leaves in horror, declaring he will wait for her at her own house.
Thaïs, alone in her bedroom, suddenly realizes the superficiality of her life. Looking into a mirror, she asks herself what will happen once her beauty fades (“Dis-moi que je suis belle”). Athanaël enters and she warns him not to love her. He replies that there is a kind of love she doesn’t know and that this love will lead to eternal life. The voice of Nicias, heard from outside, reminds Thaïs of her past. Rejecting both her life of luxury and Athanaël’s God, she collapses in despair. Athanaël says he will wait for her outside the door until daybreak.
After a night of soul-searching (Orchestral interlude: “Méditation”), Thaïs appears from her house to tell Athanaël that she is ready to follow him. He explains that he will take her to the convent of Mother Albine, but before they leave she must burn her palace and all her belongings. She agrees, asking to keep only a statue of Eros, but Athanaël smashes it and leads her back into the house. Nicias enters with his friends. He has gambled and won and now wants to keep Thaïs as his mistress a while longer. After a dancer has performed a ballet, accompanied by the singing of Nicias’s slave girls, Thaïs and Athanaël reappear from the palace. When the monk announces her conversion, the crowd threatens to stone him for taking Thaïs away. Nicias distracts them by throwing money around and Athanaël and Thäis escape as her palace goes up in flames.
Thaïs and Athanaël rest at an oasis near Mother Albine’s settlement. Thaïs is exhausted and feels unable to continue, but Athanaël demands that she ignore her physical weakness. Only when he sees that her feet are bloody does he feel pity; he goes off to get water. Thaïs praises his kindness and thanks him for having brought her to salvation (Duet: “Baigne d’eau mes mains”). At the convent, Albine and the nuns welcome Thaïs. When she says goodbye to Athanaël, he realizes with horror that he will never see her again.
Back at the monks’ retreat, Palémon remarks that Athanaël seems like a dead man since his return. Athanaël confesses that, in spite of all his fasting and prayer, the image of Thaïs’s beauty still haunts him. Palémon once again repeats his warning not to get involved with the affairs of the outside world. In his sleep, Athanaël sees Thais, first as a temptress, then as a saint about to die in the monastery. Waking, he cries he must go to her and runs into the desert and an approaching sandstorm.
Thaïs lies dying after three months of penance. Albine welcomes the distraught Athanaël, and Thaïs gratefully remembers how he saved her. The monk replies that she has converted him to worldly love. Thaïs, in a trance, doesn’t understand his passionate confession and dies with a vision of angels greeting her in heaven. Athanaël, shattered, is left alone, begging God for mercy.