The protagonist, Hanehara Hiyori, comes to the isolated island of Torishirojima during her summer vacation to sort through her late grandmother’s belongings. After getting off the infrequent ferry, she encounters a mysterious girl. The girl, with her hair tousled by the sea breeze, is gazing out at the horizon, where the boundary between the sea and sky is indistinguishable. By the time Hiyori notices, the girl has vanished, leaving Hiyori feeling as though she were playfully tricked by a fox. She proceeds to her grandmother's house, where her aunt, a relative, is already sorting through the estate.
While helping with the cleaning of her grandmother's belongings, Hiyori finds herself adjusting to the island life, something completely different from her urban upbringing. As she engages with nature in ways she never did before in the city, the island reminds her of something forgotten—memories of a long-lost time.
Throughout her stay, Hiyori meets several girls, each with their own mysteries: a girl gazing out at the sea, another searching for a strange butterfly, a third seeking memories of a pirate ship, and a fourth living quietly in a lighthouse. These encounters lead to the formation of new friendships, and Hiyori finds herself wishing that this summer would never end.