A fifth atlas - deep teal, the colour of the sea at dusk - appears on Asha's grandmother's shelf, and Asha, Kian, and Ixchel arrive on a great grey sea where each island is a language and small wooden boats carry single words between them. Their host is the Ferryman, the oldest of the Keepers, who has been carrying meaning between tongues for centuries. The Silence has learned a darker trick - sinking the boats, severing the connections between languages themselves, so that cognates lose their resonance and translation becomes impossible. With the Ferryman, his apprentice Pip, and a great-aunt Asha never knew she had, the children must find the seventh voice of restoration: the voice of cross-linguistic kinship.