This Ecchi Anime is the Sequel to High School DxD New: Teishi Kyoushitsu no Vampire OVA. Based on a light novel series by Ishibumi Ichiei with illustrations by Miyama Zero.

Hyoudou Issei has recurring dreams of the deceitful actions of his first girlfriend, Raynare the fallen angel, that cause him to question the girls around him. After another hectic morning, he discovers that the Hyoudou residence has been rebuilt as a three-story mansion including an training ground, bath and swimming pool. Summer vacation is coming up, and the Occult Research Club members are going on a trip to the underworld. They enter by means of a protected train route.

A surprise attack planned by Azazel helps determine what sort of training each member needs. And the next day, Azazel has them each start on their 20-day training session. Issei trains with Tannin, a reincarnated former Dragon King. After finishing the training, Issei is still unable to activate his Balance Breaker, so Azazel suggests the need for a catalyst for the activation...