Seventeen-year-old Cassandra Mortmain (Romola Garai) struggles to survive in a decaying English castle. Her father is desperate to repeat the spectacular success of his first novel, but hasn't written a word for 12 years; her exquisite sister Rose (Rose Byrne) can only rail against their fate; and their bohemian step-mother Topaz is a nudist, and no help at all. Salvation comes in the form of their American landlord Simon Cotton and his brother Neil. Although initially repelled by Simon, Rose is determined to make him fall in love with her and succeeds. A wedding is arranged and Cassandra is left on the sidelines as everyone around her is drawn into a maelstrom of interconnected relationships. But events spiral out of control, and before the summer ends she will experience frustrated desire, first love, and a broken heart.