The monk Athanaël returns to his religious brethren from Alexandria. He tells Palémon, the leader of the congregation, and his colleagues how shocked he was to find the city immersed in sin. Athanaël blames this spiritual disorder on the beautiful courtesan Thaïs. Athanaël recalls seeing her in the years before he had taken his vows.
When the monks retire, Athanaël has a vision of Thaïs. He determines to return to Alexandria to try to convert the courtesan, despite Palémon’s warnings not to meddle in other people’s lives.
Athanaël visits his friend, Nicias, who has paid a fortune to spend a week with Thaïs. Nicias agrees to introduce him to Thaïs, who will soon be arriving for her final night with Nicias. When she appears, she notices the monk staring at her, but dismisses his sermon. Athanaël tells her that he will come to her house the next day with the promise of salvation.
Thaïs gazes into her mirror, wondering what will happen when her beauty fades. Athanaël arrives and tells that her that the love he offers brings the blessing of eternal life and freedom from sin. Although she is fascinated by his message, the spell is broken when she hears Nicias’ voice outside, reminding her of her past. Torn between her current life of luxury and the promise of redemption, she ponders her future throughout the night. Athanaël waits outside for her decision.
The next morning, Thaïs is ready to follow Athanaël’s path to holiness. He tells her that she must set fire to her house and burn all of her worldly possessions. She agrees, hoping only to retain a statue of Eros given to her by Nicias, but Athanaël angrily destroys it. Outside, when Athanaël announces that Thaïs has consecrated herself to God, the crowd turns violent at the thought of losing her. To distract them, Nicias throws money at the mob. Thaïs and Athanaël flee to safety as her past life erupts in flames.
On their journey to Mother Albine’s convent, Thaïs and Athanaël stop to rest at a desert oasis. Exhausted, she can barely walk. He comforts her, and she thanks him for leading her to salvation. In the solitude of the oasis, they acknowledge the powerful connection that has brought them together. When they arrive at the convent, Athanaël realizes that he will never see her again.
He returns to his brethren, but cannot stop thinking about Thaïs. In dreams, he first sees her as a temptress, then sees her dying. When he wakes, he decides that he must go to her.
Athanaël sees Thaïs surrounded by the nuns who pray for her. He confesses that his love for her has always been physical desire. But she is unreachable, her soul having ascended to heaven. Athanaël begs for God’s mercy.