On Marin's island, sunrise doesn't come every 24 hours; it comes every 28 years. Now the sun is just a sliver of light on the horizon. The weather is turning cold, and the shadows are growing long. Because sunset triggers the tide to roll out hundreds of miles, the islanders are frantically preparing to sail south, where they will wait out the long night. Marin and her twin brother, Kana, help their anxious parents ready the house for departure. Locks must be taken off doors. Furniture must be arranged. Tables must be set. The rituals are puzzling — bizarre, even — but none of the adults in town will discuss why it has to be done this way. Just as the ships are about to sail, a teenage boy goes missing: the twins' friend Line. Marin and Kana are the only ones who know the truth about where Line's gone, and the only way to rescue him is by doing it themselves. But night is falling, and their island is changing. It may already be too late.