Hiroshi Akejima, son of Kōshirō Akejima and brother to Koyomi Akejima, saw his world shattered by the 2047 FOS alien invasion. This sudden attack destroyed major cities and killed his father, a military commander, profoundly shaping Hiroshi's path. By 2053, fueled by vengeance for his father's death, Hiroshi serves as a soldier and robot weapon pilot within the Planetary Defense Forces Alliance. He actively contributes to Plan Z (Planzet), humanity's desperate final strategy to retake Earth.

This critical plan requires temporarily deactivating the Diffusor, the global shield that had blocked further alien advances. During this vulnerable period, Hiroshi and fellow pilots face the perilous task of defending their base against overwhelming waves of FOS forces. His position places him directly in the path of the primary alien onslaught targeting the human resistance. Throughout these events, Hiroshi maintains his familial connection with his sister, Koyomi.