During the summer of his second year of high school, Yuushin Tahara discovers that his childhood friend, Miyu Fukagawa, has moved back into the house next door after being away for a long time. She has grown into a very cute young woman, but Yuushin quickly senses that there is something disconcerting and strange about her despite her friendly demeanor. The story is described as a friendship thriller that explores the unsettling relationship between a popular but off-kilter girl and her friend who is trying to understand what has changed.

The narrative focuses on Yuushin, a gentle and somewhat passive protagonist, and Miyu, whose quirky behavior hides a more disturbing interiority. As the series progresses, what begins as a seemingly fresh suspenseful romantic comedy about reuniting with an eccentric childhood friend soon reveals itself to be a darker psychological thriller. The plot is driven by a series of rapid twists and mounting mysteries, with reader discussions suggesting a major revelation that the real Miyu might be dead and has been replaced by someone else. The story follows the pair along with their circle of friends as they navigate the increasingly unnerving adventures and secrets that arise from Miyu’s return.