A boy named Kazuki Tsukigase arrives on the remote island of Tokishima, having left the city to escape from anyone who knows him and the secret he carries. The island is a place of lush nature, home to a location the locals call the Cove of the Disappeared, a spot rumored to be where people vanish. One day in early summer, Kazuki discovers a girl collapsed and unconscious in that very cove.

The girl is taken to the island clinic, where she is unable to provide any information about her past or her family. She gives her name as Nanao Akishika and says she is sixteen years old, but the only other clue she offers is a single whispered word: 1974. With no one to care for her, she is taken in by a local man named Takatsu, who helped rescue her alongside Kazuki. Intrigued by the mysterious girl, Kazuki begins to visit Nanao after school. As they spend time together over the summer, the distance between them slowly closes, and their daily lives begin to fill with new and unfamiliar emotions.

A classmate of Kazuki's, Mikiya Ozaki, also becomes part of the circle of people drawn into the mystery surrounding Nanao. However, just as a bond begins to form, Nanao reveals an unexpected truth to Kazuki, forcing them both to confront the reality that they were never meant to meet. Their journey explores complex emotions and hidden secrets, building toward a bittersweet future after a single transformative summer together.