Japan, 1699. Sano’s young son disappears during an Edo moon-viewing festival, and a political rival suggests that the child has been taken to Ezogashima. Sano and Reiko travel north, where Lord Matsumae is holding local people hostage until the murder of his mistress is solved.
Sano agrees to investigate in exchange for his son’s release, but the case is embedded in clan warfare, colonial authority and hostility between Japanese rulers and the island’s indigenous communities. The winter landscape and distance from Edo strip Sano of his usual power, making family survival dependent on alliances he cannot command.