Hosho Aiko should be dead. To be precise, when she came back home one day, she found her entire family slaughtered by a grotesque monster. The creature attacked and killed her as well, but she finds herself awakening in a mysterious room, taken care of by Dr. Kochigawa. She has amnesia, but when another monster appears before her in her room, she regains her memories and her left arm changes into a powerful weapon, very similar to that of the monster, which she destroys.