(spoilers)

ACT I
Alexandria and the Thebaid desert in Egypt, fourth century C.E. At a Cenobite settlement, Athanaël, a monk, returns from Alexandria with news that the city is in a state of sin. The people are besotted by Thaïs, a courtesan and actress, whose performances are causing a sensation. Athanaël admits to his fellow monks that once, in his youth, he fell under her spell. Now he considers her behavior an affront to God and is determined to convert her to a Christian life. Palémon reminds him that it is against his vows to interfere with the secular world, but after dreaming of Thaïs, Athanaël defiantly returns to Alexandria to save her soul.

Athanaël goes to the house of his old school friend Nicias, now a leading Alexandrian of extreme wealth. Nicias is skeptical of Athanaël’s chances in converting Thaïs but offers to introduce him. She is, after all, his current lover—a service he purchased at a great price but which he cannot afford to renew. That night, at the farewell party, a very public confrontation occurs between the two adversaries. Thaïs rejects Athanaël’s impertinent demands that she change her way of life and warns him against suppressing his human nature. He vows to continue his campaign for her soul. She dares him to do so and submits him to a humiliating ceremony in the name of Venus.

ACT II
Alone in her bedroom, Thaïs wearily considers the worthlessness of her life and seeks assurance both from her mirror and from Venus that her beauty will be eternal. Athanaël visits her unannounced. Her routine seduction has no apparent effect on him, but when he claims that the love he offers her will bring eternity, it resonates with her. The voice of Nicias outside reminds her of the nature of her current life. She sends Athanaël to dismiss Nicias, but, left alone, she collapses in perplexity and fear.

Athanaël waits outside for Thaïs’s decision. In due course, she appears with the news that she has made up her mind to follow him. Athanaël is overjoyed but makes it clear that the road will be hard. He demands that she destroy her home and everything in it. As they are making plans, Nicias brings a happy crowd to her door in the hope of reclaiming her for the night’s revels, but all hope of that vanishes when she and Athanaël appear at the threshold of the burning house. The citizens try violently to keep their idol, but when her determination becomes clear to Nicias, he helps Thaïs and Athanaël escape the angry crowd.

ACT III
Thaïs and Athanaël are in the desert on their way to the convent of Mother Albine. Thaïs is exhausted and broken, but Athanaël ruthlessly demands that she push on. Only when he sees her bleeding feet does he feel pity. Thaïs thanks him for having brought her to salvation. At the convent the nuns welcome her. When the door closes, Athanaël suddenly realizes what it will mean to him never to see her again.

Athanaël has been back with the Cenobites for three months. In spite of prayer, fasting, and flagellation, he is unable to drive the physical image of Thaïs from his spirit. He attempts to confess to Palémon but even here fails, and Palémon realizes that he is probably lost. That night, Athanaël has a violently erotic dream of Thaïs, and voices tell him she is dying. He decides to return to the convent to steal her away from God.

After three months of penance, Thaïs is at the end of her strength and rests in the convent garden. Her virtue and purity have been such that the nuns have already declared her a saint. Athanaël arrives too late. Thaïs is already out of his grasp as she dies in a vision of angels.