I Capture the Castle is Dodie Smith's first novel, written during the Second World War when she and her husband Alec Beesley, a conscientious objector, moved from their native England to California. Smith was already an established playwright and later became famous for writing the children's classic The Hundred and One Dalmatians.

The novel concerns an eccentric family struggling to live in genteel poverty in a decaying castle during the 1930s. The first-person narrator is Cassandra Mortmain, who tells the story through her journal. It is a coming-of-age story in which Cassandra becomes a young woman and experiences her first love.

The novel takes place between April and October in a single year in the 1930s. Ten years before, widower James Mortmain took out a forty-year repairing lease on a dilapidated but beautiful castle, hoping it would inspire him and cure his writer's block. He has been unable to write anything since the success of his first book, Jacob Wrestling (a reference to Jacob wrestling with the angel), an innovative and "difficult" modernist novel that made his name internationally. So far, his family has survived on royalties from that book, but they are dwindling to nothing. Now they are selling off the furniture to buy food.