To escape an unpleasant incident in his past, a young man named Hairi Takahara travels to the remote, rural island of Torishirojima in the Seto Inland Sea under the pretense of settling his late grandmother's estate. As he begins sorting through her belongings, he finds himself drawn into the island's peaceful summer rhythm, a world away from his urban life. He soon encounters four enigmatic girls, each connected to the island's mysteries in different ways. The theatrical compilation film distills the 26-episode television anime series into a cinematic experience, focusing on the distinct story arcs of these heroines.

Shiroha Naruse is a quiet, isolated girl often seen gazing at the sea, who seems to have forgotten her own summer vacation. Ao Sorakado is a cheerful local who chases the legendary "butterflies of memories" across the island. Kamome Kushima is a refined visitor who always carries a large, mysterious suitcase and claims to be searching for a pirate ship from a childhood promise. Tsumugi Wenders is a younger, innocent girl staying at the old lighthouse, trying to discover what she wants to do with her life.

As Hairi helps these girls, he is drawn into a series of bittersweet and supernatural events. In Tsumugi’s arc, he learns her time on the island is limited and vows to make her summer unforgettable, uncovering a decades-old spirit bound by regret. During Ao’s story, he helps her pursue the legendary butterflies, delving into the island’s forgotten cycles and history. Kamome’s journey involves confronting a melancholic, unfulfilled promise tied to a phantom ship. These seemingly separate threads all build toward Shiroha’s arc, where Hairi discovers a profound personal connection to the island and its central secret, forcing him to confront the very memories he was running from and the true meaning of a summer that never ends.