Northanger Abbey begins by introducing us to its heroine, Catherine Morland, an unexceptional but kind girl of seventeen. She has grown up in the countryside, the eldest daughter of a parson in a family of ten children. Catherine is a plain child, but gets prettier as she gets older. She also begins to care about her clothing and obsessively read novels.
Catherine is thrilled to be invited by a rich, childless couple from her neighborhood, Mr. Allen and Mrs. Allen, to take her first trip away from home. When she arrives in the vacation town of Bath, Catherine is disappointed to find that Mrs. Allen, who cares about little other than clothing, knows no one. Catherine meets a young man of twenty-four named Henry Tilney. She finds him charming and hopes to see him again soon.
Soon after, while in the Pump-room (one of the central meeting points in Bath), Catherine and Mrs. Allen meet an old classmate of Mrs. Allen’s named Mrs. Thorpe, and Catherine becomes fast friends with Mrs. Thorpe’s daughter Isabella. The Thorpes already know Catherine’s older brother James, who goes to school with Mrs. Thorpe’s son, John. Catherine and Isabella become inseparable, but Catherine continues to look for Henry Tilney, who seems to have left Bath.
One day to Catherine’s surprise, she and Isabella run into their brothers in the street. Catherine does not notice that James and Isabella have feelings for one another. Catherine is introduced to Isabella’s brother John, a rude man who talks of little but horses, but who asks Catherine to dance with him at a ball that night. Despite thinking John seems ill-mannered, Catherine has too little confidence in her own judgment to decide that she does not like him.
At the ball, John leaves Catherine to talk to a friend about horses, and James and Isabella leave her to dance together. Catherine feels that she looks as if she could not find a partner. To her surprise, Henry appears and asks her to dance. She sadly declines his offer, because she is already engaged to dance with John. John returns and they dance, but Catherine continually looks back at Henry. During the dancing Catherine meets Henry’s sister, Miss Eleanor Tilney. Catherine hopes Henry will ask her to dance again and feels crestfallen when she sees him lead another woman to the dance floor. John wants to dance again, but she refuses him.
The next day, Catherine hopes to meet Eleanor and get to know her better. Instead, John, Isabella, and James convince her go on a drive with them. Catherine rides with John, who scares Catherine by saying that her brother’s carriage is unsafe, then takes this back when she becomes alarmed. Catherine is confused by John’s self-contradiction. Returning from the drive, Catherine is upset to learn that Mrs. Allen ran into the Tilneys while she was out.
The Thorpes continue to get in the way of Catherine developing her relationship with the Tilneys. At another ball, Catherine is asked to dance by Henry, but John interrupts, saying Catherine promised to dance with him. Catherine dances with Henry anyway and has a wonderful time.
Catherine plans a walk with the Tilneys for the next day, but when it rains, she is unsure if the Tilneys will come. John convinces Catherine to go on another carriage ride by saying he saw the Tilneys driving out of town. From the carriage, Catherine sees the Tilneys walking through town. She is angry at John, who seems to have lied about seeing the Tilneys, but he refuses to stop the carriage so she can get out.
Catherine apologizes to Henry Tilney for missing their walk that night at the theater. She also sees Henry’s father, General Tilney, talking to John Thorpe and looking at her.
The next day Catherine reschedules her walk with the Tilneys for the following day, but the Thorpes and James beg her to change her plans to go on another drive. Catherine refuses, but John reschedules her walk with the Tilneys without her permission. Catherine runs to the Tilneys to take back what John has done. After an enjoyable walk, Eleanor Tilney invites Catherine to come dine with them the next night.
The next day, Isabella tells Catherine that she and James are engaged, but she worries the Morlands will not approve of her as a daughter-in-law. Catherine had not suspected their romance and is shocked and overjoyed. As Catherine is leaving the Thorpes’ lodgings, John waylays her. He talks in abstract terms about their marrying, but she hardly listens and understands nothing.
At the next ball, Catherine dances with Henry, while Isabella, who told Catherine she did not intend to dance, dances with Henry’s older brother Captain Frederick Tilney, who has just come to town. Catherine expresses surprise to Henry Tilney, who observes that Catherine does not understand other people’s motives, because she only considers how she herself would behave in any situation, and she is more good-natured than others.
The next day, Isabella learns how much the Morlands will give her and James. Isabella seems disappointed about the amount of money and suggests that Mr. Morland has not been generous. Catherine feels hurt, but Isabella says she is only disappointed that she and James must wait several years to marry.
Catherine is thrilled to receive an invitation to travel to the Tilneys’ home at Northanger Abbey, where she will live in an old building like the ones in the books she loves to read.
The next day in the Pump-room, Isabella encourages Catherine to marry John. Catherine is dumbfounded to hear that John wants to marry her, but tells Isabella that she is interested in Henry, not John. Captain Tilney then enters and sits down next to Isabella. Catherine overhears them flirting and feels jealous on James’s behalf.
In the days before Catherine’s departure for Northanger, she observes this flirtation with growing alarm. She asks Henry to tell his brother to leave Bath, but Henry says no outside interference should be needed to ensure Isabella’s loyalty to James.
Soon after, Catherine leaves Bath with the Tilneys. On the ride there, Catherine tells Henry how excited she is to go to a real abbey like the ones she has read about. Henry spins a tale about the mysterious and frightening events likely to happen to her in an old building like Northanger. Catherine is spellbound, though she knows Henry is teasing her.
Catherine hopes to uncover a mystery at Northanger. After finding nothing exceptional in her room, Catherine develops a theory that the General is a villain and murdered his wife. She sneaks alone to Mrs. Tilney’s room, where she is discovered by Henry. Learning of her suspicions, Henry urges her to be a better judge of situations in the future. She feels humiliated and sure he will never love her now, but he is only kinder to her after this.
Catherine receives a letter from James saying that his engagement to Isabella is off. He advises Catherine to leave Northanger before Captain Tilney arrives to announce that he is engaged to Isabella. Distressed, Catherine tells Henry and Eleanor about her brother’s letter, but they say their father will not approve of the marriage since Isabella has no fortune. Catherine is puzzled, because she has heard General Tilney say he does not care about money. Soon after, the General hints that he hopes Catherine and Henry will marry. Catherine hopes that Henry feels the same way.
Isabella writes to ask Catherine’s help in resolving a misunderstanding with James, but Catherine now sees through Isabella’s hypocrisy and resolves to forget her former friend.
Soon after, the General leaves for London for a few days and Henry leaves Northanger. One night the General returns unexpectedly. He sends a distraught Eleanor to tell Catherine that she is to be unceremoniously expelled from the house the next morning. Catherine is shocked, but tries to hide this from Eleanor.
A miserable Catherine returns home, where she is greeted joyfully. Her family resents the way she has been treated, but counsels that she forget about it. Catherine mopes around the house, but no one guesses that she is in love.
Three days later, Henry unexpectedly arrives at Fullerton and asks Catherine to marry him. He explains that the General was misled in Bath by John to believe that Catherine was very rich, then subsequently told by John in London that she was quite poor. The General wanted Catherine to marry Henry when he believed she was an heiress, then rushed back to expel her from Northanger when he learned she was not. Henry, to his father’s shock, refuses to obey the command to forget her.
The Morlands give their permission for Catherine and Henry’s marriage on the condition that the General give his. Eventually, after Eleanor marries a rich Viscount, and the General learns that Catherine is not as poor as he had been led to believe, he gives his permission, and Catherine and Henry are married.