On the day of the Tanabata festival, Shidou Itsuka happens to meet the eerie Kurumi Tokisaki, who proceeds to ask him out on a date. Although nervous, Shidou accepts the offer. The two spend their time visiting to a planetarium, buying food at the festival, and trying out a mock wedding ceremony. Meanwhile, Shirou's friends—Tooka Yatogami, Yoshino, and Kotori Itsuka—are looking for him, while also enjoying the festival themselves.
As Shidou spends the day with Kurumi, he begins to notice an unexpected softer side of her. He knows that Kurumi is infamously called the "Worst Spirit," but the girl standing before him is a gentle soul. However, as the day draws to a close, Shirou finally begins to understand the truth behind it all.
Summary taken from: MyAnimeList