The head of the Mortmain family is Cassandra Mortmain’s father, a writer suffering from writer's block who has not published anything since his first book, a hit entitled Jacob Wresstling. Ten years before the story begins, he took out a 40-year lease on a dilapidated but beautiful castle, hoping to find either inspiration or isolation there; now, his family is selling off the furniture to buy food.

Mortmain's second wife, Topaz, is an exotically beautiful artist's model who enjoys communing with nature, sometimes wearing nothing but hip boots. Rose, the elder daughter, is a classic English beauty longing for a chance to meet some eligible (and preferably rich) young men. Cassandra, the younger daughter and the story's narrator, has literary ambitions and spends a lot of time developing her writing talent by "capturing" everything around her in her journal. Stephen, a handsome, loyal, live-in son of the Mortmain's late cook, and Thomas, the youngest Mortmain child, round off the cast of household characters.

Stephen, a noble soul, is in love with Cassandra, which she finds touching, but a bit awkward; Thomas, a schoolboy, is, like Cassandra, considered "tolerably bright". Things begin to happen when the Cottons, a wealthy American family, inherit nearby Scoatney and become the Mortmains' new landlords. Cassandra and Rose soon become intrigued by the unmarried sons, Simon and Neil, and the inevitable relationships commence.

As the journal advances, the relationships Cassandra depicts become subtler and more problematic, and she concludes her narrative on a bittersweet note. Cassandra, despite being saddened by her first disappointment in love, can still reflect with satisfaction on the Mortmain family's improved fortunes, and looks forward to the future.

I Capture the Castle is a classic coming-of-age novel which stands the test of time and will endure for as long as boy-girl relationships endure.