The narrator wants to explain why Mrs. Fitzpatrick has gone from, first, refusing ever to speak to Tom, to now, inviting him to her house.
The day before, Mrs. Fitzpatrick had visited both Squire Western and Mrs. Western.
They were rude and horrible to her.
She now realizes that the Westerns will neverforgive her for eloping.
So she decides she wants revenge.
Mrs. Fitzpatrick thinks back to her own history.
She remembers that Mrs. Western used to be in love with Mr. Fitzpatrick.
And she thinks that, now that Mrs. Western is older and Tom is so good-looking, Mrs. Western would definitelyfall for him if she had the chance.
So Mrs. Fitzpatrick suggests to Tom that he try and flirt with Mrs. Western.
This will give him an opportunity to meet with Sophia in secret.
Tom thinks this is a great plan, but he points out that Sophia really hates lying, and feels a huge sense of obligation to her aunt.
Mrs. Fitzpatrick answers angrily that her aunt is a "hagg" (16.9.8) (yes, she uses two "g"s—that's how much she hates Mrs. Western) who deserves to be punished for all the harm she's done in this life.
Tom sees what Mrs. Fitzpatrick really wants to do (hurt Mrs. Western), and he also knows this plan hasn't got a chance of working.
So he keeps thanking Mrs. Fitzpatrick and refusing the idea.
Finally, he says that Sophia is so far above him that it seems wrong for someone as low as Tom to pursue her.
Mrs. Fitzpatrick hears these compliments about Sophia and starts applying them to herself.
In fact, Mrs. Fitzpatrick now feels herself falling under Tom's spell.
Mrs. Fitzpatrick answers that any woman who doesn't see how awesome Tom is (*ahem*Sophia*ahem*) is unworthy of him.
Tom suddenly starts to suspect that Mrs. Fitzpatrick may be into him herself.
He quickly excuses himself.
Mrs. Fitzpatrick says she'll be home all day tomorrow, if he wants to come by and (*koff*) talk over her plan.
Tom thanks her and takes off.
He is so focused on Sophia that he isn't thinking about other women (which makes a change!).