The Red Dragon Emperor, Issei Hyoudou, and the Occult Research Club return to action as summer break arrives for the students of Kuoh Academy. Following their battle with Issei’s sworn enemy, Vali, and the Chaos Brigade, it becomes evident just how inexperienced Rias Gremory's team is. Consequently, she and Azazel put the club through a rigorous training regimen in the Underworld to ready them for the challenges ahead. As they gradually mature as a team, Issei once again finds himself in intimate situations with the girls of the Occult Research Club. Meanwhile, their adversaries grow stronger and more numerous as they rally their forces. And with the sudden appearance of Loki, the Evil God of Norse Mythology, the stage is set for epic battles and wickedly powerful devils in High School DxD BorN!