Shouta Handa, at the age of 25 and currently unemployed, is fed up with his dull existence. In search of inspiration, he attends a class reunion, only to find that his peers have achieved remarkable success. Feeling disheartened by his own lack of progress, he steps outside for a smoke to clear his head. In a twist of fate, he accidentally falls into the school swimming pool, triggering a supernatural phenomenon that sends him back ten years. He emerges in the midst of a swimming class, surrounded by his female classmates in bikinis. Now, as a mature man in a youthful form, how will Shouta cope with their allure and intimacy?