Setting: Byzantine Egypt (late fourth century, A.D.)
The cenobite monk Athanaël returns to his religious brethren after travelling to Alexandria. He explains to his fellow monks and to Palémon, the leader of the congregation, how shocked he was to find the city immersed in salaciousness and sin. Athanaël blames this spiritual disorder on the courtesan Thaïs. Athanaël recalls meeting Thaïs, a priestess in the cult of Venus, years earlier, before he had taken his vows.
The community retires to rest. Athanaël has a dream in which he sees Thaïs naked. He decides that he must return to Alexandria to try to redeem the courtesan. In spite of Palémon’s warnings not to meddle in other people’s lives, the monk sets off through the desert to the city of sin.
In Alexandria, Athanaël visits a friend from his youth, Nicias, who is infatuated with Thaïs and has squandered a fortune on her. The monk explains the mission that has brought him to the city once again, but Nicias laughs, believing Athanaël will never succeed. Nonetheless, Nicias agrees to introduce him to Thaïs, who will be coming to his house for dinner that very evening. When she appears, the monk disapprovingly stares at her. Thaïs is somewhat disconcerted when she hears his sermon, but she invites him to visit her at her house if he hopes to convert her.
In her bedroom, Thaïs looks at herself in the mirror, wondering what life will be like when her beauty fades. Athanaël arrives and tries to persuade her to follow his doctrine, through which she will be blessed with eternal life and will be freed from sin and death. At first his words horrify Thaïs, but little by little she is overcome by an internal peace and she begins to feel happy. From outside the voice of Nicias is heard, calling to her. Athanaël says that he will wait outside for her at dawn. Throughout the night Thaïs meditates along about the spirituality of her soul.
The next morning, Thaïs is ready to follow the path of holiness. Athanaël will lead her to a convent, but she must first destroy all of her worldly possessions by fire. When Athanaël tells the crowd in the plaza that Thaïs has consecrated herself to God and that she will be leaving with him, they jeer and make ready to stone her and the monk. Nicias throws some gold coins on the ground to distract the crowd, as Thaïs and Athanaël flee to safety.
Thaïs and Athanaël travel through the desert to a monastery run by Mother Albine. She is exhausted and barely has the strength to walk. The monk explains that this suffering is the penitence she must endure to rid herself of her sins. But when he sees that Thaïs’ feet are bleeding, he feels sorry for her and goes in search of water. He kisses her feet and comforts her. She thanks him for his kindness and for saving her soul. Once in the monastery the nuns welcome Thaïs. She bids the monk farewell and kisses his hands with love and admiration. Athanaël, knowing he will not see her again, falls apart.
Athanaël once again returns to his brethren. Twenty days have passed and he is unable to stop thinking about Thaïs; he now realizes that he desires her with physical passion. He then has a dream in which he sees Thaïs dying in the monastery courtyard. When he awakens, he decides that he must see her again.
Albine welcomes Athanaël, who finds the weakened Thaïs surrounded by the devout nuns. Athanaël tells her that his feelings for her are physical love, desire. In a state of mystical ecstasy, she is oblivious to the monk’s words. She passes away peacefully, leaving Athanaël in despair.