Sousuke Sagara, a teenage soldier in the covert anti-terrorist organization Mithril, is ordered to go undercover as a high school student to protect Kaname Chidori. She unknowingly possesses "Whispered" abilities, knowledge of advanced Black Technology that multiple factions want to capture. Having grown up on battlefields, Sousuke treats every school situation as a potential combat scenario, clashing with the headstrong Kaname at every turn while real threats close in from outside.