Having miraculously survived a terrorist attack that nearly destroyed the world's most advanced submarine, the Tuatha de Danaan, Mithril opens a hunt for the traitor, both to avenge their fallen comrades and to shake out all possible information about the mysterious enemy. Meanwhile, Sagara Sousuke is torn between his job as a mercenary and battle-mecha pilot and protecting Chidori Kaname while posing as a high school student. But as backup, an elusive intelligence agent with the codename "Shadow" has already been keeping an eye on Kaname for some time. And Sousuke receives an unexpected order concerning the girl he is assigned to protect...