In the MMORPG Cross Reverie, Sakamoto Takuma is so powerful that he is lauded as the "Demon Lord" by other players. One day, he is summoned to a world outside his own - but with the same appearance he had in the game! There, he meets two girls who both proclaim themselves to be his Summoner. They perform an Enslavement Ritual to turn him into their Summon... but that's when Takuma's passive ability <> activates! Instead, it is the girls who become enslaved! Though Takuma may be the strongest Sorcerer there is, he has no idea how to talk with other people. It is here he makes his choice: to act based on his persona from the game! "Amazing? But of course... I am Diablo, the being known and feared as the Demon Lord!" So begins a tale of adventure with an earth-shakingly powerful Demon Lord (or at least someone who acts like one) taking on another world!
Takuma Sakamoto was a shut-in gamer who achieved renown as the Demon Lord in Cross Reverie, a MMORPG, for his overwhelming gear and skill.
After awaking to hands filled with breasts, Diablo follows Shera and Rem to register as new members of the Adventurer's Guild. While Shera is identified as a level 30 Summoner by the guild, Diablo's level proves too high for them to measure.
The Warrior Emile, defender and ally of all women, challenges Diablo to a fight.
Rem, Celestine, and even Emile struggle in their battle against Gregore's sudden attack. Meanwhile, Diablo and Edelgard confront each other on the Bridge of Ulug.
The Kingdom of Greenwood demands that Faltra City hand over Princess Shera. If they refuse, it will lead to war, so Diablo meets with Galford, the Lord of Faltra City, and accepts a quest to prevent war from breaking out.
While Greenwood's demands remain unmet, Diablo and the girls visit a slave market to see if they can learn the magical secret of the Enslavement Collars.
The following day after Prince Keera's visit, Shera suddenly bids farewell to Diablo while they're preparing potions. Unable to stop her from leaving, Diablo holes up in the inn, consumed with regret. Rem is unsettled to see him so negative, too.
Diablo squares off with Prince Keera's Summon, the Force Hydra, a monster he has never faced before.
Alicia returns to the Imperial Capital to give her report on the incident between Faltra and the Kingdom of Greenwood. There she encounters the Paladin, Saddler.
The night of the Demon Lord's resurrection arrives at long last. Rem finally reveals to Shera and Alicia that the Demon Lord Krebskulm's soul is sealed within her. Together, Diablo's party heads for Starfall Tower, where Edelgard awaits.
Diablo's party returns to Faltra with the newly resurrected Klem. On their way back from visiting Celestine, Rem and Klem suddenly get separated from Diablo and Shera, with Alicia having led them both astray.
The Demon Lord Krebskulm has awakened due to Alicia's betrayal. Diablo stands alone against Klem, her mind consumed by hatred. Shera races to Rem's side. An army of Fallen led by Edelgard marches on Faltra, and the curtain rises on their final battle.