During the Tanabata festival, Shidou Itsuka encounters the mysterious Kurumi Tokisaki, who surprises him by inviting him on a date. Despite his initial apprehension, Shidou agrees and they explore various festival attractions together, including a planetarium visit, sampling festival treats, and participating in a pretend wedding. Meanwhile, Shidou's friends—Tohka Yatogami, Yoshino, and Kotori Itsuka—search for him amidst their own festival adventures. As Shidou spends time with Kurumi, he uncovers a side of her that contradicts her notorious title as the 'Worst Spirit,' revealing a kinder persona. As the day progresses, Shidou gradually unravels the complex truth hidden beneath Kurumi's facade.