Three months have passed since Iori Kitahara began living above Grand Blue, his uncle's scuba diving shop, and joined his university's Diving Club. Alongside the other members, he has spent his days drinking and recklessly having fun. However, when Iori's sister, Shiori, hears about his behavior, she decides to take matters into her own hands—and bring him back home to make him take over their family's inn. Despite her best efforts, Shiori temporarily gives up as she sees how Iori has grown attached to Grand Blue and his friends. As Iori and the rest of the Diving Club continue to party like there is no tomorrow, they end up in all kinds of bizarrely amusing situations, still somehow managing to hone their scuba diving skills!