A shy gamer unwittingly assumes the role of his high-level demon lord character, Diablo, when two of the game's characters summon him into their world; when the summoning spell backfires on the two girls, the trio head to the nearest city for help.

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 in a challenge against Emile, a chivalrous but ignorant warrior; Diablo reluctantly decides 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 their stalemate drags on.

The Greenwood kingdom threatens war if Faltra City does not return its princess, Shera, and even puts a bounty out 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 as the Kingdom of Greenwood's threat looms over them.