Orgon's family is up in arms because Orgon and his mother have fallen under the influence of Tartuffe, a pious fraud (and a vagrant prior to Orgon's help). Tartuffe pretends to be pious and to speak with divine authority, and Orgon and his mother no longer take any action without first consulting him.
Tartuffe's antics do not fool the rest of the family or their friends; they detest him. Orgon raises the stakes when he announces that he will marry Tartuffe to his daughter
Mariane (already engaged to Valère). Mariane, of course, feels very upset at this news, and the rest of the family realizes how deeply Tartuffe has embedded himself into the family. In an effort to show Orgon how awful Tartuffe really is, the family devises a plan to trap
Tartuffe into confessing to Elmire his desire for her. As a pious man and a guest, he should have no such feelings for the lady of the house, and the family hopes that after such a confession, Orgon will throw Tartuffe out of the house. Indeed, Tartuffe does try to seduce Elmire, but their interview is interrupted when Orgon's son Damis, who has been eavesdropping, is no longer able to control his boiling indignation and jumps out of his hiding place to denounce Tartuffe.

Frontispiece and titlepage of "Tartuffe or The Imposter" from a 1739 collected edition of his works in
French and English, printed by John Watts. The engraving depicts the amoral Tartuffe being deceitfully seduced by Elmire, the wife of his host, Orgon who hides under a table.

Tartuffe is at first shocked but recovers very well. When Orgon enters the room and
Damis triumphantly tells him what happened, Tartuffe uses reverse psychology and accuses himself of being the worst sinner:
Oui, mon frère, je suis un méchant, un coupable.
Un malheureux pécheur tout plein d'iniquité
(Yes, my brother, I am an evildoer, a guilty man,
An unhappy sinner full of iniquity) (III.vi).
Orgon is convinced that Damis was lying and banishes him from the house. Tartuffe even gets Orgon to order that, to teach Damis a lesson, Tartuffe should be around
Elmire more than ever. As a gift to Tartuffe and further punishment to Damis and the rest of his family, Orgon signs over all his worldly possessions to Tartuffe.
In a later scene, Elmire takes up the charge again and challenges Orgon to be witness to a meeting between herself and Tartuffe. Orgon, ever easily convinced, decides to

hide under a table in the same room, confident that Elmire is wrong. He overhears, of course, Elmire resisting Tartuffe's very forward advances. When Tartuffe has incriminated himself beyond all help and is dangerously close to violating Elmire,
Orgon comes out from under the table and orders Tartuffe out of his house.
But this wily guest means to stay, and Tartuffe finally shows his hand. It turns out that earlier, before the events of the play, Orgon had admitted to Tartuffe that he had possession of a box of incriminating letters (written by a friend, not by him). Tartuffe had taken charge and possession of this box, and now tells Orgon that he (Orgon) will be the one to leave. Tartuffe takes his temporary leave and Orgon's family tries to figure out what to do. Very soon, Monsieur Loyal shows up with a message from
Tartuffe and the court itself – they must move out from the house because it now belongs to Tartuffe. Dorine makes fun of Monsieur Loyal's name, mocking his fake loyalty. Even Madame Pernelle, who had refused to believe any ill about Tartuffe even in the face of her son's actually seeing it, has become convinced by this time of
Tartuffe's duplicity.
No sooner does Monsieur Loyal leave than Valère rushes in with the news that Tartuffe has denounced Orgon for aiding and assisting a traitor by keeping the incriminating letters and that Orgon is about to be arrested. Before Orgon can flee, Tartuffe arrives with an officer, but to his surprise the officer arrests him instead. The officer explains that the enlightened