One blustery night in 1928, a whale washes up on the shores of the English Channel. By law, it belongs to the King, but twelve-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, kitchen maid; Taras, 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, and her imagination comes to life.

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, it turns out, and one that threatens to tear the family apart.