Misaka No. 20002 (Original heroine) has reached the level of LV5. Due to the genetic mutation of the "real body" Misaka Mikoto, her appearance and abilities are completely different from other Misaka sisters and Misaka Mikoto, and she also has her own consciousness. So, lab staff gave her a brand new name. Her last name is not Misaka, her last name is Tokisaki. As one of the genetically mutated clones, her responsibility is to protect the final clone, Misaka No. 20001. Because of a programming error, she personally killed the extraneous individual. Those self-righteous experimenters believed that the program was absolutely fine and expelled No. 20003. They happened to meet Accelerator who had just killed Misaka No. 9982, And the story of the two began...