Misaka is one of the the 7 strongest people in Academy City, a "scientific city of the future" of students learning to control their special abilities. In this story she deals with an esper enhancement program run slightly amok along with protecting her younger sister.. This is the second Railgun and fourth story in the entire overarching story line -- you should watch Index 1 and 2 first and you really should watch Railgun 1 to help understand more about things.