In a world where nine different races coexist, humans ended up at the bottom of the hierarchy. Despite facing discrimination daily, a young human named Light leaves his family in hopes of becoming a great adventurer. While registering as one, however, Light ends up becoming a mere F-Rank adventurer due to his unimpressive skill, Unlimited Gacha, which allows him to materialize a randomly rolled card into existence.

Light is soon recruited to become the human representative of the Concord of the Tribes, a party that intends to represent all of the nine races. As the party ventures out to the Abyss, the world's toughest dungeon, his party suddenly decides to abandon him in its depths. Just as his death seems certain, Light rolls a Level 9999 card that summons a beautiful and strong maid named Mei.

Swearing her loyalty to Light, Mei sets out to eliminate his betrayers. Now nearly invincible with Mei, alongside other incredible summons, Light sets out to get his revenge.