After the final exams, everyone in the noticeably looser atmosphere of the school is planning and looking forward to summer vacation. Yujin, a sincere and good model student, is completely absorbed in an exchange diary. The exchange diary is one that is passed back and forth daily through an empty locker, and the identity of the partner is kept secret. Yujin develops feelings for that partner and tries to arrange a meeting before summer vacation starts, while Sang-kyu grows increasingly worried about Yujin…