Seventeen-year-old Catherine Morland is one of ten children of a country clergyman. Although a tomboy in her childhood, she is "in training for a heroine"[5] and is fond of reading Gothic novels "provided they [are] all story and no reflection."[6]
The Allens (her wealthier neighbours in Fullerton) invite Catherine to accompany them in their visit to the city of Bath and partake in the winter season of balls, theatre and other social activities. Shortly after their arrival, she is introduced to a young gentleman, Henry Tilney, with whom she dances. Mrs. Allen meets an old school friend, Mrs. Thorpe, whose daughter, Isabella, quickly becomes friends with Catherine. Isabelle introduces Catherine to Ann Radcliffe's 1794 Gothic novel Mysteries of Udolpho. Mrs. Thorpe's son, John, is a friend of Catherine's older brother, James, at Oxford University where they are both students. The two young men come to Bath, where John is then introduced to Catherine.
The Thorpes are not happy about Catherine's friendship with the Tilneys. They correctly perceive Henry as a rival for Catherine's affections even though Catherine is not at all interested in John Thorpe. Despite Thorpe continually attempting to sabotage her relationship with the Tilneys, Catherine tries to maintain her friendships with both the Thorpes and the Tilneys. This leads to several misunderstandings, which put Catherine in the awkward position of having to explain herself to the Tilneys.
Isabella and James become engaged. James' father approves of the match and offers his son a country parson's living of a modest sum, £400 annually, but they must wait until he can obtain the benefice in two and a half years. Isabella is dissatisfied, but to Catherine, she misrepresents her distress as being caused solely by the delay, and not by the value of the sum. Isabella immediately begins to flirt with Captain Frederick Tilney, Henry's older brother. Innocent Catherine cannot understand her friend's behaviour, but Henry understands all too well as he knows his brother's character and habits.
The Tilneys invite Catherine to stay with them for a few weeks at their home, Northanger Abbey. Once at Northanger Abbey, Catherine and Eleanor Tilney, Henry's and Frederick's younger sister, get to know each other better on a personal level.[7] Catherine, in accordance with her novel reading, expects the house to be exotic and frightening. Henry teases her about this as it turns out that Northanger Abbey is pleasant and decidedly not Gothic. However, the house includes a mysterious suite of rooms that no one ever enters; Catherine learns that they were the apartments of Mrs. Tilney, who died nine years earlier due to a serious illness,[7] leaving Mr. Tilney with three children to raise by himself.[8] As General Tilney no longer appears to be affected by her death, Catherine decides that he may have imprisoned her in her chamber, or even murdered her.
Catherine discovers that her over-active imagination has led her astray as nothing is strange or distressing in the apartments. Henry finds and questions her; he surmises and informs her that his father loved his wife in his own way and was truly upset by her death.[9] She leaves the apartments, crying, fearing that she has lost Henry's regard entirely. Realising how foolish she has been, Catherine comes to believe that, though novels may be delightful, their content does not relate to everyday life. Henry does not mention this incident to her again.
James writes to inform her that he has broken off his engagement to Isabella and implies that she has become engaged instead to Captain Tilney. Henry and Eleanor Tilney are sceptical that their brother has actually become engaged to Isabella Thorpe. Catherine is terribly disappointed, realising what a dishonest person Isabella is. A subsequent letter from Isabella herself confirms the Tilney siblings' doubts and shows that Frederick Tilney was merely flirting with Isabella. The General goes off to London, and the atmosphere at Northanger Abbey immediately becomes lighter and more pleasant from his absence. Catherine passes several enjoyable days with Henry and Eleanor until the General returns abruptly in a temper in Henry's absence. He forces Catherine to go home early the next morning in a shocking and unsafe mode that forces Catherine to undertake the 70 miles (110 km) journey alone.
At home, Catherine is listless and unhappy. Henry pays a sudden unexpected visit and explains what happened. General Tilney (on the misinformation of John Thorpe) had believed her to be exceedingly rich as the Allens' prospective heiress, and therefore a proper match for Henry. In London, General Tilney ran into Thorpe again, who, angry at Catherine's refusal of his earlier half-made proposal of marriage, said instead that she was nearly destitute. Enraged, General Tilney, (again on the misinformation of John Thorpe), returned home to evict Catherine. When Henry returned to Northanger, his father informed him of what had occurred and forbade him to think of Catherine again. When Henry learns how she had been treated, he breaks with his father and tells Catherine he still wants to marry her despite his father's disapproval. Catherine is delighted, though when Henry seeks her parents' approval, they tell the young couple that final approval will only happen when General Tilney consents.
Eventually, General Tilney acquiesces because Eleanor has become engaged to a wealthy and titled man; he discovers that the Morlands, while not extremely rich, are far from destitute.
