Now that Frosine, Élise, and Mariane are gone, Harpagon asks Cléante what he thinks of Mariane. Cléante replies that she's just okay: sort of pretty, sort of smart, but nothing special. Basically, he lies. He lies like a rug.
Harpagon says that really is a shame, because he was about to suggest that Cléante marry Mariane instead of himself.
Cléante answers that if this is what Harpagon really wants, he'll agree to it. He's a good son like that. But Harpagon says no, no, no—he doesn't want to force Cléante into marrying a woman he doesn't love.
This goes on for a little while before Cléante breaks down and admits that he loves Mariane and has already talked about marriage with her during his many visits to her house.
Harpagon then turns to the audience and says that this is what he's been trying to figure out all along. Now that he knows Cléante has been working against him, he orders his son to give up trying to marry Mariane. Harpagon has decided to marry her and that's final. Daddy's word is law.
Cléante, super-angry at being tricked by his devious old Dad, tells Harpagon that there's no way he'll ever give up on marrying Mariane.
They argue over who should get Mariane, and Harpagon gets so fed up that he calls out for his whoopin' stick.