As summer vacation approaches for Kuoh Academy students, the Red Dragon Emperor, Issei Hyoudou, and the Occult Research Club are back in action. Rias Gremory's crew is clearly inexperienced after their struggle with Issei's sworn adversary, Vali and the Chaos Brigade. As a result, she and Azazel take the club through an arduous training regimen in the Underworld to prepare them for the next difficulties. While they gradually evolve as a team, Issei will find himself in intimate settings with the females of the Occult Research Club once more. Meanwhile, their foes become more powerful and numerous as they gather their troops. With the unexpected apparition of Loki, the Evil God of Norse Mythology, the stage was set.