Class 3-4 fought through the final battle against two major assassin organizations at the end of the school year and won. But then, they learned of the true mastermind behind it all and an even greater threat: Soji Kodo and the Kodo Family. Now, summer vacation has arrived. Arakuni Ibuki enjoys a brief respite, but soon after, a shocking truth is discovered: the email from the “traitor” was not sent by Kuran Shuto… A mysterious person in Class 3-4, “X,” suddenly appears!!