One blustery night in 1928, a whale washes up on the shores of the English Channel. By law, it belongs to the King, but 12-year-old orphan Cristabel Seagrave has other plans. She and the rest of the household --- her sister, Flossie; her brother, Digby, long-awaited heir to Chilcombe manor; Maudie Kitcat, the kitchen maid; and Taras, a visiting artist --- build a theatre from the beast’s skeletal rib cage. Within the Whalebone Theatre, Cristabel can escape her feckless stepparents and brisk governesses. As Cristabel grows into a headstrong young woman, World War II rears its head. She and Digby become British secret agents on separate missions in Nazi-occupied France --- a more dangerous kind of playacting that threatens to tear the family apart.