One day, the protagonist, a salaryman, is suddenly reincarnated as a “humanoid giant weapon”. Not the pilot. Instead, it’s a half-biological, half-mechanical weapon that stands over ten meters tall and weighs several dozen tons. To his horror, the protagonist is unable to move his body of his own will, nor can he speak. His body obeys the will of the pilot, and at the pilot’s command he confronts the enemy’s humanoid weapon, only to have his limbs blown off by a giant sword. In a cold apocalyptic world where everything is frozen, the hero and other “Titans” are the only superweapons that can operate. The protagonist plays an active role as an ace machine, but he is always wounded, and the mechanic girl who repairs his body is his only source of comfort. However, a massive attack by enemy forces causes him to lose the girl right in front of his eyes. Enraged, the hero “awakens” as a Titan who moves with free will.