After surviving a series of chaotic events, Mithril mercenary sergeant Sousuke Sagara and the "Whispered" high school student Kaname Chidori resume their ordinary school routines. Sousuke persists in his attempts to blend in with civilians while simultaneously handling his missions, whereas Kaname tries to assist him (despite his bizarre behavior frequently frustrating her). Nevertheless, as their daily lives stabilize, fresh issues emerge. The emergence of an independent terrorist group called Amalgam, along with internal strife within Mithril, threatens to shatter their cohesion into a chain of failures and betrayals. Furthermore, the introduction of new, potent Arm Slave models exacerbates the crisis when global peace hangs in the balance. As Kaname and Sousuke confront their internal struggles amid life-threatening combat, will they manage to make the correct decisions and alter their destiny?