After surviving a series of calamitous events, mercenary sergeant Sousuke Sagara of Mithril and "Whispered" schoolgirl Kaname Chidori return to their usual high school life. Sousuke continues his efforts to integrate with the general populace while still managing his task, while Kaname tries to assist him (even though his antics often drive her up a wall). However, as their lives begin to settle, new difficulties emerge. The emergence of an independent terrorist group known as Amalgam, as well as internal divisions within Mithril, threaten to destabilize the organization and lead to a sequence of failures and betrayals. When the world's peace is at stake, new, powerful Arm Slave models only exacerbate the issue. Will Kaname and Sousuke face their own inner conflicts while fighting for their lives?