After a mysterious rift between dimensions opens, Arata Kasuga and the Trinity Seven are pulled into an unfamiliar world—one teeming with devils, fallen angels, and sacred artifacts. Drawn by the overwhelming magical disturbances, the group arrives at Kuoh Academy, immediately attracting the attention of Rias Gremory and her peerage. Mistaking Arata and his allies for dangerous invaders, Rias challenges them, leading to a fierce but inconclusive battle. However, the misunderstanding is soon resolved when both sides realize they share a common enemy: a powerful alliance of fallen angels, rogue magicians, and ancient supernatural entities who seek to collapse the barriers between worlds. This faction, led by a fallen angel lord rivaling Azazel, aims to harness forbidden grimoires and corrupted sacred artifacts to dominate both realms. With their goals aligned, the Trinity Seven and the Occult Research Club forge a fragile but growing alliance, training together to prepare for the escalating threat. Lilith teaches Rias advanced gun magic, Levi and Koneko sharpen their close-combat skills, and Arin’s barrier magic complements Asia’s healing powers, blending their fighting styles in ways none of them could have achieved alone. Meanwhile, Arata and Issei form an unbreakable friendship, their shared perverted nature often lightening the mood amidst the rising tension.

Their battles intensify as they face waves of fallen angels, corrupted exorcists, and terrifying magicians wielding ancient, reality-breaking spells. The enemy's forces grow stronger, unleashing demonic dragons fused with fallen angels and warriors empowered by forbidden grimoire magic. Arata’s chaotic spellcraft and Demon Lord powers collide with the precision of the fallen angels' light-based attacks, while Issei’s rapidly evolving Boosted Gear pushes him to new, explosive levels of strength. As their friendship deepens, the two unleash devastating combination attacks, ripping through enemy strongholds with unstoppable force. The final battle takes place at the heart of the collapsing rift, within a pocket universe where the fallen angel lord wields a weapon forged from the corrupted pages of forbidden grimoires. The two sides clash in a cataclysmic showdown—Akeno and Lilith’s joint elemental storms tear through enemy legions, Koneko and Levi devastate the flanks, and Xenovia’s swordsmanship fuses with Mira’s high-level spells to shatter the enemy’s defenses. Pushed to their limits, Issei taps into his Juggernaut Drive while Arata fully embraces his Demon Lord transformation, their combined power overwhelming the fallen angel lord in a final, desperate assault. With the rift sealed and the multiverse stabilized, the two groups part as close allies. As Arata and his friends prepare to return to their world, he smirks at Issei, promising they’ll definitely meet again—preferably somewhere with even more beautiful girls.