Warning: Contains significant plot spoilers
Catherine Morland, the daughter of a rural clergyman, is a spirited tomboy with a passion for Gothic novels, which she devours in secret in the garden of her countryside home.
When family friends Mr. and Mrs. Allen suggest they take Catherine to Bath for the season, Catherine can barely contain her excitement. She imagines elaborate balls filled with handsome men and beautiful ladies enjoying the latest dances. And, from the moment her carriage arrives at the opening ball of the season, she is not disappointed.
It is in Bath that she makes the acquaintance of the Thorpe family. Vivacious Isabella Thorpe immediately takes the nervous Catherine under her wing and delights in showing her the highlights of the season, including the fun of flirting with boys and sharing gossip. Isabella is engaged to Catherine's brother James, making her the fashionable older sister Catherine has always wished for.
Isabella's brother John soon declares his romantic interest in Catherine. With his confident swagger and flirtatious persistence, John makes it clear he has designs on Catherine as a wife. However, Catherine's interest is in Henry Tilney, son of the intimidating General Tilney. She has the good fortune to gain the General's approval, which is founded upon the exaggerated report of her family's wealth - given to him by the misinformed John. The General invites Catherine to come and stay at Northanger Abbey, the medieval family home of the Tilneys, and Catherine excitedly accepts, again projecting her reading of gothic novels on to the eerie place.
Meanwhile, Isabella has noticed how handsome Henry's suave older brother Captain Frederick Tilney is. Driven by her desire to be a wealthy wife, she breaks off her engagement with James and allows Frederick to seduce her. Yet Isabella is one of many pretty young girls the dashing Captain has seduced, and he has no intention of marrying his latest conquest.
At Northanger Abbey, Henry's gentle sister Eleanor takes Catherine under her wing and, along with Henry, Catherine finds herself happiest with her new friends. Catherine and Henry become close until, somewhat unbalanced by an assiduous reading of Radcliffe's horrifying The Mysteries of Udolpho, Catherine imagines a mystery surrounding the death of the General's wife several years earlier. Henry catches her looking for evidence of a murder in the locked rooms of Northanger Abbey and is offended by her mistrust in his family.
General Tilney, having now realized Catherine's parents are more humble than wealthy, forbids Henry to have any further romantic thoughts of her. Catherine is expelled from the house, devastated that she has offended her hosts with her suspicions. Yet Henry cannot get Catherine out of his mind. Disobeying his father, he follows Catherine to her home to reveal his true feelings.