Following their tumultuous past, mercenary sergeant Sousuke Sagara from Mithril and the 'Whispered' school girl Kaname Chidori come back to their ordinary high school routines. While Sousuke strives to blend in with the civilian world and handle his covert mission, Kaname supports him despite being exasperated by his antics. Yet, as their lives settle down, new challenges emerge. The rise of a rogue terrorist group named Amalgam and internal rifts within Mithril jeopardize their unity, leading to a cascade of failures and betrayals. The introduction of advanced Arm Slave models further heightens the global peril for peace. Amidst life-threatening battles, Kaname and Sousuke grapple with personal demons, facing crucial decisions that could reshape their destinies.