Summer break is coming up fast for Hyodo Issei, but all his plans are put on hold when he finds out he is to accompany Rias for her visit back home to the underworld! A new adventure is about to begin and all the while Koneko is not acting herself.

Issei and the others begin their training to improve each of their skills. All the while things are happening around them as powerful people gather to fight against the Chaos Brigade. All the young devils gather in one place.

Koneko's sister Kuroka has appeared to take her sister away from Rias Gremory. Issei and Rias chase after her to stop that from happening. At the same time, a figure who objects to these alliances between mythologies appears at the great gathering.

Loki has been sealed away, but only temporarily. Rias and her family are among the volunteers trying to hold him back, while Odin returns North to retrieve the powerful hammer Mjolnir. Will everyone be able to stand up and fight against a god?

With Loki defeated, Rias and her household return to the human world to finish up their last days of summer. A pile of homework awaits Issei, but he goes on a date with Akeno instead! Akeno throws herself at Issei, hoping to forget her terrible past.

Issei and the others are back to school and their daily lives. Irina and Rossweisse both start to live at Issei's house and the sports festival is fast approaching. During all of this, Diodora Astaroth appears before Asia and proposes to her?!

Diodora continues to pester sweet Asia as the Rating Game approaches. Issei and the others prepare for battle as dark things brew in the background. When the Gremory household jumps to the field, they find themselves in an unexpected battle!

Rias and her household leaves the fighting of the Chaos Brigade to the adults as they rush toward the temple to save Asia. There, Diodora replaces the Rating Game with a little game of his own. They must fight against his forces to get Asia back!

Asia saves Issei by healing the dragon within him after his incomplete transformation. Finally, they all make it back home and enjoy their peaceful days. After Loki fails with Issei, he turns his evil grasp toward Rias.

Issei goes to see Vali for help, which is in itself an action against the underworld. With the help of Arthur and his Holy Royal Sword Collbrande, the Rias household goes to the Dimensional Gap. There, they meet Rias and try to bring her back.

Issei must stop Rias by fighting against her. As she pummels him with attacks, Issei recalls various memories that he shared with her. Unable to leave Rias in such a state, he runs forward to bring back the regal and dignified Rias he remembers.