In the early 1950s, an eleven-year-old boy boards a huge liner bound for England - a 'castle that was to cross the sea'. At mealtimes, he is placed at the lowly 'Cat's Table' with an eccentric group of grown-ups and two other boys, Cassius and Ramadhin. As the ship makes its way across the Indian Ocean, through the Suez Canal, into the Mediterranean, the boys become involved in the worlds and stories of the adults around them, tumbling from one adventure and delicious discovery to another, 'bursting all over the place like freed mercury'. And at night, the boys spy on a shackled prisoner - his crime and fate a galvanizing mystery that will haunt them forever.