The eccentric Mortmain family have been rattling around in a vast, decrepit castle for years, gradually slipping into financial ruin. Mortmain is crippled by writer's block, while his second wife Topaz - a fading beauty - struggles to be a dutiful stepmother to Rose, Cassandra and Thomas. Rose needs a husband, Thomas an education but Cassandra lives for her writing. Through her gloriously witty and shrewd diaries, she chronicles the trials of family life, the impact on their prospects of the arrival of a glamorous American family and the agonies of falling in love for the first time.