In the semi-fictional world of the 1990s, the Cold War between the United States and the Soviet Union is still going on. At this time, a humanoid mobile soldier called "ArmSlave" has become the main force of the army, and the new technology related to it is coveted by various forces. The mercenary military organization "Mithril" has a powerful AS army and is responsible for combating international terrorism. Sergeant Sosuke Sagara (voiced by Tomokazu Seki) who belongs to the SRT unit under it is strong and brave on the battlefield. One day, he was assigned by his superiors to go to Japan with his teammates Melissa and Cruz to perform a protection mission. This time, their protection target is the energetic high school girl Chidori Kaname (voiced by Yukino Satsuki). Kaname is straightforward and courageous, and is the vice president of the school's student union. She has a headache about Sosuke who appears as a transfer student in Jindai High School. Because the battle master Sosuke is old-fashioned and has no common sense in his daily life, he often does surprising things, which makes the campus life of the two people turbulent and full of jokes. However, Xiaoyao hides important secrets about AS, and her future with Sosuke is destined to be full of thorns... This work is adapted from the super popular light novel of the same name by Gato Shoji.