Most good stories start with a fundamental list of ingredients: the initial situation, conflict, complication, climax, suspense, denouement, and conclusion. Great writers sometimes shake up the recipe and add some spice.
Initial Situation
We meet Catherine Morland her family, and their neighbors, the Allens. Catherine travels to Bath with the Allens and meets the Tilneys and the Thorpes.
This part of the novel screams initial situation – the stage is being set with all the necessary characters in the same place so that comedy and confusion can ensue. We meet nearly all the major players in the novel here, and we also get a good deal of expository detail about Catherine's personality and the histories of the three major families featured in the book – the Morlands, the Tilneys, and the Thorpes.
Conflict
Catherine is plagued by John Thorpe and has a series of misunderstandings with the Tilneys.
Love triangle drama ahoy. Catherine is falling for Henry, but is plagued by John and his overbearing attempts at courtship. Conflict extends to the friendship front at well, as Catherine's new BFF Isabella works with her brother to maneuver Catherine away from the Tilneys. The conflict is really hammered home by the fact that Catherine spends a large portion of this section stressing about misunderstandings or apologizing for them.
Complication
Isabella gets engaged to James Morland, but quickly begins flirting with another man: Captain Tilney. Meanwhile, Catherine travels to Northanger Abbey with the other Tilneys, but is increasingly disturbed by the overly attentive General Tilney.
Ah, the plot thickens. Isabella's seemingly fortuitous engagement is complicated by her disappointment with her future husband's income. It's complicated even further by her flirtation with Captain Tilney. Another seemingly good event also goes awry here. Catherine gets to go visit the home of her crush, Henry, and her new friend Eleanor. But General Tilney is really starting to make her nervous.
Climax
Catherine makes a huge and faulty assumption regarding General Tilney and learns a major lesson about mistaking fiction for reality.
Catherine learns a lesson. The climax is a bit of a moral one – that doesn't mean it's not exciting. Catherine accuses General Tilney of being a murderer to Henry, albeit in a round about way. Probably not the best thing to tell your crush, but there you go. Henry calls her on her behavior and Catherine realizes the error of her ways. Catherine's entire trajectory in Bath was colored by her faulty assumptions and the ways in which she mistook fiction for reality. In the aftermath of her confrontation with Henry, Catherine grows up a lot and becomes much more self-aware. Lesson learned, but Catherine's personal relationship with Henry is cast into some doubt after her blunder, temporarily at least.
Suspense
The scandalous truth about Isabella's relationship with Captain Tilney comes out; her engagement to James, and her friendship with Catherine, is broken. Catherine is expelled from Northanger Abbey by General Tilney.
Even though Catherine has learned some valuable lessons, a number of tense situations still have to be resolved. Catherine and Henry's relationship seems like it's still fine. But Catherine, Henry, and Eleanor weather a tense couple of days after learning about Captain Tilney and Isabella's relationship. Just as this situation is resolved, Catherine is expelled from Northanger Abbey in extremely mysterious circumstances.
Denouement
Catherine returns home to Fullerton. Henry follows her and proposes.
Things are finally winding down and resolutions abound here. Catherine returns home in disgrace, only to be followed by Henry, who puts her out of her misery by proposing. A happily ever after conclusion seems to be in the works, but there's still the problem of the disapproving General Tilney.
Conclusion
General Tilney consents to Henry's marriage, and Catherine and Henry finally marry.
We finally learn why General Tilney overreacted to Catherine's presence in his house. Eleanor fortunately marries a rich man and, since the General has one rich in-law, he doesn't mind having a less rich one. So Catherine and Henry get married.