This was the first novel that Jane Austen wrote, and it's all about the trials and tribulations of Catherine Morland, Austen's young heroine. At the start of the novel, Catherine is living a run-of-the-mill life in a small village. However, when a couple of family friends invite her to the city, she jumps at the chance. This opens up a new social life, as she becomes best friends with a girl called Isabella, and the two spend their time going to the hottest social events and raving about the latest literary craze: the Gothic novel, with its mix of horror, intrigue, romance, and spooky settings.
Despite attempts to set her up with Isabella's brother (cue a series of comic misunderstandings), Catherine falls for a guy called Henry Tilney and makes friends with his sister, too. It's no surprise, then, that she agrees to visit their home, Northanger Abbey. Having recently read Anne Radcliffe's The Mysteries of Udolpho, one of the most popular Gothic novels of its day, Catherine is keen to embark on a Gothic adventure of her own.
Okay, so Catherine gets disappointed pretty quickly when she finds that Northanger Abbey isn't the mysterious Gothic ruin that she had imagined. The imagination is a powerful thing, though, and it's a major reason why literature can be so gripping. Take ghost stories: we may find ourselves more aware of creaking floorboards or the whistling of the wind if we've just read a chilling tale. Once we're in that mind-set, it doesn't always take much to fire up the imagination… and that's what happens to Catherine.
The seeds are planted when Catherine starts to suspect that General Tilney's (that's Henry's dad) strange behavior and mood are due to him having, you know, murdered his wife. She starts snooping around, and of course she uncovers some kind of manuscript and starts picturing the terrible secrets it might conceal. She spends the rest of the night—a dark, stormy night—imagining weird sounds and possible intruders.
When daylight arrives and she reads the manuscript, however, she finds that it's a—drum roll please—laundry list. Austen is pretty much poking fun at the conventions of Gothic novels, including The Mysteries of Udolpho, in which the heroine becomes preoccupied with the grisly sights that may lie beneath a black veil… only to find that all that's really there is a wax statue.
In narratological terms, we'd call this "intertextuality," as Austen references the devices that characterize the Gothic genre. As for Catherine, she finally comes to realize that these novels don't reflect real life.