In 1945, World War II came to an abrupt and unexpected end when an alien force known as the Phantom Beasts appeared on Earth and began slaughtering the human population. Within years, more than half the planet was under their control. By 1999 — fifty-four years into the conflict — humanity is still fighting for survival. Earth's forces now field advanced mecha called Humanoid Walking Tanks to combat the Phantom Beasts, but the toll on pilots has been catastrophic. To keep the HWT program staffed, the Japanese government implemented a student draft, conscripting high school students as pilots. Gunparade March follows the 5121th Platoon — a unit composed mainly of those drafted teenagers — as they struggle to fight an alien war while trying to preserve something resembling a normal life.