Years ago, astronaut Byakuya left behind precious materials in the belief that his son, the scientifically-gifted Senkuu, would one day find them. In the present, the Kingdom of Science's Kohaku and Ginrou have infiltrated the harem of the Petrification Kingdom’s leader with the help of Amaryllis, a local girl. At first, their mission progresses without a hitch. Unfortunately, an error enacted by Kohaku and Ginrou leads Minister Ibara and the Island’s strongest warrior, Moz, to identify the intruding duo as enemies. Even worse, Moz tails Amaryllis when she returns to inform Senkuu of the news, discovering the scientist's secret hideout. However, Moz's agenda does not align with Ibara's—he wants to dethrone the minister and keep the petrification device for himself. If Senkuu plays his cards right, he could gain a powerful ally and win the war once and for all.

After the battle with the Witch Cult, Subaru travels back to Crusch's manor with Emilia, when he discovers, to his horror, that everyone has forgotten Rem. Earlier, she and Crusch were ambushed by the Sin Archbishops of Gluttony, Lye Batenkaitos, and Greed, Regulus Corneus. Although they fought fiercely, both ended up having their existences eaten by Lye. This caused Rem to fall into a coma, with noone but Subaru remembering her, and Crusch lost her memories. After learning what happened, a devastated Subaru commits suicide in hopes of going back in time to before the ambush to prevent it. However, it fails as he returns to after the ambush had already occurred. Subaru later attends a meeting with Emilia, the amnesiac Crusch, Felis and Wilhelm. After a heated discussion, they decide to continue their alliance. After the meeting, Emilia comforts Subaru as he grieves for Rem.

A decade ago, mankind witnessed the deadly potential of weapons caused by the Great War. To avoid another catastrophe, governments worldwide have resorted to espionage to fulfill their agendas. In the Din Republic, Lily is an enthusiastic young girl who aspires to become a spy and serve her nation. However, she begins to doubt her abilities, as she struggles to perform well academically. Lily's passion is reignited when she is offered provisional graduation by joining the spy team Lamplight, which she eagerly accepts despite the enigmatic nature of the opportunity. Upon arriving at her destination, Lily is surprised to find six other girls waiting there, all of whom have faced their own academic difficulties as well. A mysterious man named Klaus soon appears and reveals they must complete an "Impossible Mission": to infiltrate the Galgad Empire after just one month of training. Although skeptical, the girls eventually agree to work together under Klaus' guidance—as this might be their only chance to prove their worth as spies.

The misadventures of Issei Hyoudou, high school pervert and aspiring Harem King, continue on in High School DxD New. As the members of the Occult Research Club carry out their regular activities, it becomes increasingly obvious that there is something wrong with their Knight, the usually composed and alert Yuuto Kiba. Soon, Issei learns of Kiba's dark, bloody past and its connection to the mysterious Holy Swords. Once the subject of a cruel experiment, Kiba now seeks revenge on all those who wronged him. With the return of an old enemy, as well as the appearance of two new, Holy Sword-wielding beauties, it isn't long before Issei and his Devil comrades are plunged into a twisted plot once more. [Written by MAL Rewrite]