The story is set in 1963 in Yokohama. Kokuriko Manor sits on a hill overlooking the harbour. A 16 year-old girl, Umi, lives in that house. Every morning she raises a signal flag facing the sea. The flag means “I pray for safe voyages”. A 17 year-old boy, Shun, always sees this flag from the sea as he rides a tugboat to school. Gradually the pair are drawn to each other but they are faced with a sudden trial. Even so, they keep going without running from facing the hardships of reality.