Having been saved by his classmate, Kuchina Hitsujimoto, Sousei decides he has to thank her. However, what starts out as a letter of gratitude eventually develops into an exchange diary also involving Asumi Shiramori, Hina Shigee, and Yukisada Hayashi. Kuchina ends up enjoying her first exchange diary, yet she can't help but feel pain whenever she has fun. Conditioned into thinking this way by the boss of the All-Japan Assassin Association, Furuhiko Kagemiya, she's forced to choose between spending time with her friends and peace...