Seventeen-year-old Cassandra Mortmain lives in a crumbling ruin of a castle in the Suffolk countryside. Her father, an author wrestling with writer's block, spends his time reading detective stories; her bohemian stepmother Topaz is an artist's model, and her beautiful older sister Rose is determined to escape the poverty of their existence – ideally by marriage to a rich man. Intelligent and observant, Cassandra chronicles the eccentricities of her relations in her journal, in a bid to capture her family and their lives at the castle.


When two rich American brothers, Simon and Neil Cotton, arrive unexpectedly, the family's isolation is disrupted. Rose accepts a marriage proposal from Simon, and as she is whisked off to a world of wealth in London, Cassandra is left to deal with the increasingly bizarre behaviour of her father – as well as her own feelings for Simon...

Dorothy "Dodie" Smith (1896 -1990) was an English children's novelist and playwright, best known for her novel The Hundred and One Dalmatians. Other works include I Capture the Castle, and The Starlight Barking. The Hundred and One Dalmatians was adapted into a 1961 Disney animated film version. Her novel I Capture the Castle was adapted into a 2003 a film version. I Capture the Castle was voted number 82 as 'one of the nation's 100 best-loved novels' by the British public as part of the BBC's The Big Read (2003).