About two months have passed since the hijacking incident. Sousuke Sagara, a member of the anti-terrorist organization Mithril, continues his undercover high school life while serving as Kaname Chidori’s bodyguard. But one day, a new terrorist group known as A21 begins operating in secret, setting its sights on Tessa, the young female captain of Mithril’s submarine. As the threat grows, Sousuke and his allies find themselves drawn into yet another dangerous conflict.