People can activate so-called Esper skills through medication or hypnosis. Touma Kamijou, meanwhile, is only mildly confident in his own abilities. It is the strongest Esper ability of all, but in addition to this initially advantageous aspect, it has a number of drawbacks. He simply has a lot of bad luck all the time, to put it briefly. He is disliked with the ladies, frequently challenged to a duel by other espers, and hence, is always evading capture.
However, after meeting a young girl named Index, Touma's life is drastically altered. Index is a nun at the Academy City church. She is not your typical nun, though; she has read more than 103,000 volumes. Touma quickly understands that these books are not your typical books. Additionally, he learns that certain groups are unscrupulous killers who want to get Index's knowledge and will kill people to do so.
As a result, Touma and Index's fates entwine, and before he realizes it, he is caught up in a massive adventure.
