A human boy named Light is kicked out from the adventurer party called the “Race Union (gathering of races)” by his trusted comrades. There are six races in this world: human, beast, dragon, elf, dwarf, and demon race. The human race is looked down and discriminated against in this world as the "most inferior.

"Legion (the Race Union), is a party with various race members gathered to dispel such discrimination. But, they have a hidden purpose behind it. They were ordered by countries other than human races to search for "Master," and take it under their own country. Light, a human boy, was welcomed into the Legion because of his gift "Infinite Gacha," a gift that might be owned by "Master." But in the end, their investigation showed that Light is not "Master."

Light was about to be killed by the Race Union members, but accidentally he steps on a "Transfer Trap." As a result, he was teleported to the lowest level of the dungeon and escaped from Race Union's members. In the deepest dungeon, he was attacked by a mythical demon of LV1000. In a desperate situation, Light hits the "Infinite Gacha" repeatedly. Luckily, he draws the SUR card "Mei the Seeker Maid" level 9999. She helps Light to repel the demons and saves his life.

After finally escaping from the crisis, Light decides to take revenge on his former comrades and fight against each nation to find the truth that almost killed him. Mei advises him to increase the number of trustworthy friends with "Infinite Gacha" to take revenge and find the truth. Light agreed with her words and chose to build up his power to oppose the nations in the deepest dungeon.

Three years later. Light has built up the strongest nation in the depth of the dungeon. Once again, he will return to the surface to take revenge on his former comrades and search for the truth that nearly killed him like an insect.