Beginning in 1945, following the conclusion of the Pacific War, extraterrestrial invaders infiltrated the planet's airspace, presenting an unparalleled menace that humanity had never before encountered. For the first time in recorded history, cultures across the globe united under a singular cause. Now spanning over half a century, this conflict has claimed countless lives, leaving the Japanese military no choice but to entrust the fate of the nation to the likes of Atsushi Hayami and his comrades of Unit 5121 - a group of young students. These courageous new faces soldier on, aided by HWT humanoid combat machinery and the formidable PBE bomb.