In the aftermath of the Pacific War in 1945, the world faced a dire menace as alien intruders dominated the skies, compelling humanity to unite like never before. Spanning over five decades, this ongoing conflict has claimed innumerable lives, prompting Japan's military to entrust the defense to youths like Atsushi Hayami and his classmates in Unit 5121. Armed with HWT humanoid combat machines and the powerful PBE bomb, this youthful cohort valiantly continues the fight against the extraterrestrial threat.