After being summoned to another world that looks almost exactly like the MMORPG he once played, Takuma Sakamoto now spends his days going on adventures with Rem Galleau, the black-furred Pantherian, and Shera L. Greenwood, the Elf princess, both of whom accidentally became enslaved... to him!? One day, he is suddenly approached by the Fallen, Edelgard, with a request: Would he help her to undo the seal that keeps the Demon Lord Krebskulm inside Rem? Takuma, under the guise of his in-game character Diablo, hesitates, as he had previously made a promise to Rem to help her defeat Krebskulm-- But he pushes through with his Demon Lord role-play and actually agrees! "I will revive the Demon Lord Krebskulm--and defeat them!" Now it's his Demon Lord role-play vs. an actual Demon Lord!