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..
Tartuffe's antics do not fool the rest of the family or their friends; they detest him. The stakes are raised when Orgon announces that he will marry Tartuffe to his daughter Mariane (already engaged to Valère). 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.