Tōma Kamijō is chased by hooligans after he stops one of them from disturbing a girl. In reality he was trying save him as the girl is Level 5 Psychic Mikoto Misaka, who can control and generate electricity. After losing them, he meets Mikoto who wants a duel but he refuse citing he just a Level 0 psychic. Nevertheless, she "shoots" an electrical bolt at him but it is stop by Tōma right hand which has a power called Imagine Breaker. Angered, she attacks with a powerful lighting strike which only ends up causing a citywide blackout. The next morning, after lamenting his recent bad luck, he finds a young nun hanging outside of his veranda. After feeding her spoiled food, she introduces herself as Index, a nun from the Church of England and is being chase by magicians for she "possess" 103,000 grimoires. After both of them explain each other about their own worlds, Index doesn't believe that Tōma's right hand can nullify other powers while he doesn't believe magic exist. To test their proofs, Tōma use his powers on Index's magical garb which is unfortunately blown into pieces. After biting Tōma in revenge and fixing her garb, Index concludes that his powers are the cause of his bad luck, leaves and thanks him and he tells her to come again if she needs help. After coming back from summer classes, he meets Mikoto again who wants a rematch but when he refuse again, she stomps her foot in anger causing nearby electronics to go haywire including cleaning bots who sounds the alarm which forces both of them to quickly runaway. When he returns home, he finds Index laying on the ground bleeding and behind him a man who calls himself a magician.
