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. One could even say Orgon has a single-minded obsession with Tartuffe, as clearly demonstrated in Act I, Scene 5.
The rest of the family and their friends are not fooled by Tartuffe's antics and detest him. The stakes are raised when Orgon announces that he will marry Tartuffe to his daughter Mariane (already engaged to Valere). Mariane is, of course, 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.
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.