This volume begins plot after the 3rd season of the anime (Railgun T). The Jailbreak Challenge brings back many secondary characters and introduces some new. There is a lot of humor here, making this a light read. The end sets us up for more serious doings later on.
You get two for the price of one here; the story in this volume is complete and enjoyable all by itself. It is also setting up a larger story for the two remaining volumes.