Gu Wanqing and her husband, Zhang Kai, go on a seaside vacation, enjoying a particularly pleasant time together. After applying sunscreen, Zhang Kai takes his blue surfboard into the sea for some fun, and the two agree that he will return early. But two hours pass, and her husband, who went into the sea, still has not returned or made contact. Anxious, Gu Wanqing asks around, but no one has seen him. The staff immediately dispatch speedboats to search the entire area, yet the vast sea is completely empty, and Zhang Kai has vanished without a trace. Their joyful vacation suddenly turns into a mystery. Amid the boundless blue sea, where on earth has he gone?