Kamijo meets Index, who is on the run from a magic society that wants the library of forbidden knowledge stored in her memory. Later, Kamijo is shocked to find Index at his house – bleeding and accompanied by a strange man.

Kamijo learns more about Index's history with Necessarius, the Church of Necessary Evil. Later, after a trip to the bathhouse leads to an argument with Index, Kamijo finds himself in the company of a mysterious stranger.