The story is set in Academy City (学園都市, Gakuen Toshi), a technologically advanced city located in west of Tokyo that is known for its educational and research institutions. Tōma Kamijō is a student in Academy City whose right hand, the Imagine Breaker, has the ability to negate any supernatural power, but also negates his own luck, much to his chagrin. One day, Tōma meets a young English girl named Index – a nun from Necessarius, a secret magic branch of the Church of England, whose mind has been implanted with the Index Librorum Prohibitorum – 103,000 forbidden magical books that the Church has stored in secret locations.