Fifty years ago, countless Kaiki seeds flew in from space. The world established defensive lines to counter the monstrous Kaiki, and humanity barely managed to survive.

Time passed, and now, Shinohami Ai, having completed two years of basic training with the United Human Forces, was heading to the front lines with her comrades when they were attacked by Kaiki. In this attack, her entire unit was annihilated.

Abandoned by her allies and with no rescue team in sight, she was left alone at the far end of the front line, in enemy territory. There, she discovered someone on the verge of death from a Kaiki attack and risked her own life to save them.

However, the one she saved, at the cost of suffering a fatal injury herself, was Nosferatu, the king of the monstrous Kaiki. To survive, Nosferatu merged with Ai's body.

Thus, a new soldier was born with the power of immortality, destined to become a hero - reviving stronger each time they die.