17-year-old Catherine Morland (Felicity Jones) is invited to vacation in Bath with her neighbors, wealthy and childless Mr. & Mrs. Allen.
While there, Catherine meets Isabella and the girls become close friends. Both of their brothers arrive from school; while crude John develops a crush on Catherine, her own brother James is clearly falling for Isabella, who returns the interest and they become engaged.
But Catherine has also met siblings Henry and Eleanor Tilney, and she finds Henry a pleasant contrast to the increasingly obnoxious John. As James and Isabella spend more time together, John lies to Catherine and puts her in awkward situations that force rude behavior toward the Tilneys. Henry and Eleanor understand Catherine's explanations and are gracious enough to forgive her. Their brusque father, General Tilney, invites Catherine to their home, Northanger Abbey; Catherine is delighted, expecting an abbey to be as full of intrigue and romance as the gothic novels she reads.
Henry's older brother Frederick arrives just as James leaves Bath to seek his parents' permission to marry. Frederick pays much attention to Isabella and this irritates Catherine, since Isabella flirts openly with him. James sends word that he has permission, but that he will be given a living of 400 pounds per year in 2 years' time, so the engagement will be a long one. Isabella is very unhappy with this, but maintains that it is only the delay, not the amount of money that has her distressed.
Frederick and Isabella stay in Bath while Catherine travels with the Tilneys to Northanger. The house evokes many sinister images from her beloved novels and Catherine is almost disappointed that there isn't some mystery to be solved. General Tilney leaves on business and the atmosphere becomes more comfortable. Catherine sneaks into the late Mrs. Morland's rooms -after being told that General Morland doesn't allow it- and Henry finds her there. She is shamed by her behavior and her imagination, and Henry leaves angrily.
Inconsolable, Catherine then receives a letter from James explaining that his engagement to Isabella is off since she had slept with Frederick, who then discarded her. Eleanor explains that Frederick is like that but that Henry is the exact opposite, and Catherine is brokenhearted that Henry has left. During the night, General Tilney returns and demands that Catherine pack and leave at once, which she feels she deserves although Eleanor is mortified.
After the Allens and Catherine return home, Henry arrives to see her and forgives her imaginative snooping. He explains that John and Isabella were under the mistaken impression that the Allens would one day confer their wealth upon James and Catherine, hence their interest. John had even told this to General Tilney, who had hoped to make a financially advantageous match for one of his own sons, and the General had her thrown out when he learned of her lack of fortune. Henry admits that his father married for money and broke his mother's heart with his coldness and harsh nature. Henry tells her that he and Eleanor have broken all ties with their father due to his treatment of Catherine, and Henry proposes. (Of course she accepts - this is Jane Austen material.)