Shy gamer Takuma Sakamoto must assume the role of his high-level character, demon lord Diablo, when two of the game's characters summon him into their world.

Diablo, Shera, and Rem register with the Adventurer's Guild and embark on their first quest into a dangerous forest, where Shera senses an ambush lying in wait for them.

Diablo finds himself challenged by Emile, a chivalrous but ignorant warrior. Then, Diablo reluctantly agrees to defend a fortress from a horde of beastly Fallen after realising the low-skilled soldiers don't stand a chance.

Emile defends Rem and Celestine from a Fallen infiltrator immune to their attacks. Diablo looks for a way to repel Edelgard, the Fallen horde's leader, as the stalemate drags on.

The Greenwood kingdom threaten war if Faltra City doesn't return Princess Shera and even place a bounty on her, but Shera wants to stay with Diablo. The city lord assigns an overly obedient imperial knight to aid the group.

Diablo, Shera, and Rem visit a slave market to find a way to remove the girls' slave collars while the Kingdom of Greenwood's threat looms over them.