During the Tanabata festival, Shidou Itsuka unexpectedly encounters the enigmatic Kurumi Tokisaki, who promptly invites him on a date. Though apprehensive, Shidou agrees. Their outing includes a planetarium visit, festival food, and a mock wedding ceremony. Meanwhile, Shidou's companions—Tohka Yatogami, Yoshino, and Kotori Itsuka—search for him while also enjoying the festivities themselves.
As Shidou spends the day with Kurumi, he starts to perceive an unexpected gentler side of her. He is aware that Kurumi is notoriously known as the "Worst Spirit," yet the girl before him appears kind-hearted. However, as the day draws to an end, Shidou finally begins to grasp the truth behind it all.