Lin Yan and his wife Shen Yu run a cliffside guesthouse. For half a month, she has been mysteriously measuring the reefs and invited five alumni who avoid the topic of drowning to stay for free. An astronomical tide traps everyone on the isolated island, and bizarre occurrences drive them to a breakdown. Lin Yan, with a background in physics, decodes the tricks, sees through his wife's layout for revenge over a death five years ago from failing to rescue, and chooses to silently assist. After the truth is revealed, he learns that his wife's precise timing wasn't just for revenge, but to calculate a zero-seawater homecoming path for her sea-fearing sister, completing redemption.