She did it by revisiting three of the novel's most ridiculous characters: John and Isabella Thorpe and Captain Frederick Tilney. As it is at least 20 years since the close of NA the latter is now Colonel Tilney, but he is still vain, self-centered, and inclined to coast on his name. Although he inherited Northanger Abbey a few years ago, he hasn't spent any time there, finding the countryside boring. When he meets up with John Thorpe in Bath, he takes his old acquaintance's word that the Abbey is in need of repairs, and takes him up on his offer to oversee the work.
Of course, if he had been as familiar with John Thorpe as we are, he would have known the man was prone to exaggeration and just as unlikely to do any real work as he was to tell the truth. But by Chapter 3, Thorpe is ensconced at the Abbey, where his sister Isabella, the impoverished widow Firth, soon joins him, having been turned out of her other siblings' homes. After throwing over James Morland because he wasn't as rich as she had thought, Isabella married a wealthy scion of a Scottish family; she only discovered after the wedding that he had been disinherited. Now she waits for her son Roland to finish his schooling and help support her. Roland, however, feels obligated to continue tutoring the young Charles Ballard, the crippled (but treasured) son of an industrialist. When Isabella decides she must have her son's company, both he and Charles join the group at Northanger Abbey.
The party is complete when Colonel Tilney's daughter Paulina arrives, along with her "uncle" James Morland, a reverend. Hijinks ensue: Thorpe's attempts at uncovering and rebuilding the Abbey's old chapel are increasingly ridiculous; Charles decides to explore an underground passage by himself, injuring his one good hand; and Isabella, once she finally recognizes the bearded James, makes another play for him—at least, until Paulina is called to join her family because her mother is deathly ill. Then Isabella envisions herself as the future mistress of the Abbey—she caught Tilney's attention once before, didn't she?—and ignores him forthwith. The only people who act sensibly are Roland and Paulina, who of course fall in love.
Things come to a head when the Colonel finally discovers what has been going on at the Abbey, and rushes back with Paulina to put a stop to it. He is ready to have Thorpe arrested, but Isabella provides proof the Colonel gave permission, and the situation is defused by Charles's practical father, who also joined the party. Still, it means all the "uninvited guests" must leave: Thorpe to his own unfinished project; Isabella to another sibling's house (still unwed, as the Colonel's wife recovered her health and James Morland his senses); and Roland and Charles to the seaside. All that remains is to remove any obstacles between the young lovers, and these are solved when Roland receives a letter informing him that he has come into a large trust fund from his late grandfather.
The plot isn't anything spectacular, but the fun lies in the way the author uses the characters, especially Thorpe and Isabella. They are so clueless about their own ridiculousness that it's a lot of fun to read about them even if they're not doing very much. So even though the tone of the novel is completely different from Austen's original, this sequel gets a thumb's up from me.