Catherine wants to see the old bedroom of the late Mrs. Tilney, but she cannot see it as long as General Tilney is around. There is no time to try and sneak in, because it is Sunday, and everyone has to attend a morning and afternoon church service, and lunch in between. Catherine notices that the Tilney family pew has a memorial to Mrs. Tilney, but this does not reduce her suspicions that the General killed his wife. She decides that if the guilty General could erect such a monument, he could bear to stare it at it each week.
The next day, when the General goes for one of his usual walks, Catherine sees her chance. She asks Eleanor to take her to the room, and Eleanor agrees. Just before they enter, the General appears and calls out to Eleanor. Catherine runs to her room in fright. When she recovers, she goes downstairs to find the General and Eleanor entertaining company. Pleased that she was not caught—or, at least, that the General was not angry—Catherine decides to investigate again later.
Catherine wants to settle the matter before Henry returns from his home in Woodston, so she sneaks out alone around four o'clock in the afternoon to Mrs. Tilney's old bedroom. When she gets there, she makes a shocking discovery: the room contains nothing of interest. The late Mrs. Tilney's room is actually part of the new wing of the Abbey, and there is nothing mysterious about it.
Disappointed, but still suspicious of the General, Catherine heads back to her room. On the way she is caught by Henry, who has returned early. He asks Catherine some questions, and from her reluctant, honest responses he soon guesses what she has been up to. He tells her the true story. Both Frederick and Henry had been present during their mother's illness and death, and their mother's passing had indeed hurt General Tilney. Henry then scolds her for letting herself think such horrible thoughts. Ashamed and embarrassed, Catherine runs to her room in tears.
Catherine is certain she has ruined all chances for romance with Henry. After sobbing in her room for a half hour, she goes downstairs to dinner. Henry acts no different than usual, except that he pays a bit more attention to her. This makes Catherine feel a little better, and she is soon capable of analyzing herself. She realizes that she suffered from a "voluntary, self-created delusion, each trifling circumstance receiving importance from an imagination resolved on alarm." Catherine blames the whole fiasco on the Gothic novels she had read at Bath. She realizes she is living in modern England, not the imagined world of novelist Anne Radcliffe, and that she is safe. Having reasoned herself back to the world of rational thinking, Catherine feels better, and her attitude improves as Henry acts more gallantly toward her than ever. Henry never mentions the incident.
Catherine receives a letter from her brother James, telling her that he and Isabella Thorpe are no longer engaged, and implying that Isabella is to be engaged to Frederick Tilney. Catherine does not tell Henry and Eleanor at first, but from a few hints, Henry easily guesses the truth. He does not believe Frederick will really marry Isabella until he reads the letter, and even after reading it, he remains skeptical. Henry laments his brother's apparent loss of sanity. He suggests that Isabella is only in it for the money. Henry asks Catherine whether she feels bad at the loss of a friend like Isabella, since she can hardly remain friends with her now. But Catherine finds, to her mild surprise, that she is hardly upset at the prospect of losing her friend.
Catherine's paranoid fantasy about Mrs. Tilney's murder is amusing and disturbing. Her theories are worrisome; at least in the Gothic novels she reads, there really are bad things going on. In Catherine's world, the bad things she imagines do not really exist. Northanger Abbey does not have a Gothic novel's terrible people, acts of violence and cruelty, and fog-shrouded castles and crumbling abbeys. It is a realistic coming-of-age story, and we worry for Catherine in these chapters as an imaginative frenzy consumes her. After she is chastised by Henry, Catherine understands what she has done, and vows never to let her imagination run away with her again. She does not blame the novels for her behavior, but recognizes the difference between reality and fantasy.
Henry quickly guesses at Catherine's sinister theories, and soundly admonishes her for dreaming them up. He watches her closely as he scolds her, and pays special attention to her afterward. He does not hold a grudge, and he seems sensitive to the idea that his lecture might have hurt her feelings.
Catherine also begins to realize that real people have an "unequal mixture of good and bad" in them. Her realization reflects the fictional world of Austen, in which characters behave like real people, not cardboard cutouts. General Tilney is not tender with his children, and may have been harsh to his wife, but he loved her and treated her with respect. The fact that Catherine realizes this marks a major step in her growth. She sees that even "good" people can have bad traits and that things are not as black-and-white as they are in Gothic novels. This realization is a loss of innocence, but it is also a gain in maturity.