The setting of "The World 21" takes place in a world known as "The Land of Gloom", where the generally accepted "currency of power" is a type of soul shard made into chips. The protagonist also owes an unwieldy debt of soul shards. However, suddenly, the ancient god "Verus" chooses the protagonist to become a "debt collector", granting him the power to control poker to collect all souls loaned by Verus but not yet returned.

The poker cards have been endowed with mysterious power, capable of summoning various powerful creatures and spells to assist the players in battle. Each card represents a creature or spell from The Land of Gloom, all with their unique characteristics and abilities. Players need to use strategy and luck to draw and use these cards to gain an advantage in combat.