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In Sakurazaka's imaginative literary world, the United Defense Force has been fighting Mimics, a nightmarish advance force sent by a race that wants to remake the earth into a habitat in which they can live. That means humanity has to die.
Keiji, a rookie warrior, sustains a horrible injury in his first battle. Rita Vratiski, a virtual goddess of destruction reverentially known as the Full Metal B****, finds him as he bleeds out. He manages to ask her if this is the end. ”Yeah, “ she tells him, “But I’ll stay with you until you die.” And die he does.
Then he wakes up. Through a convergence of unlikely events, he is fated for this to happen over and over again. Every time he dies, he loops back in time. He soon realizes he can train in such a way that he might be able to eventually win the battle – and perhaps the war – on behalf of humanity. He takes courage from the mindset of the samurai: “Strike down the enemy, and learn.” He shakes off his brooding nihilism, stops experimenting with ways to die, and chooses to fight.