Fate Graphite lives miserably as a gatekeeper in a world where skills given at birth determine an individual's entire life. Afflicted by the seemingly harmful skill Gluttony, which makes him perpetually hungry, he is abused by the holy knights—the supposed protectors of the populace—who view him as a dreg of society due to his skill. However, Fate's destiny takes a dramatic turn after he kills an escaping injured thief. To his surprise, Gluttony activates, enhancing his stats and granting him new skills.

Following this epiphanic event, Fate receives a job offer from Roxy Hart, the only benevolent holy knight in the city, which he happily accepts. Eager to further explore Gluttony's true potential, he partners with a mysterious talking sword named Greed, who warns him that Gluttony is a taboo skill that breaks the logic of the world. Now forced to live a double life, Fate must slay monsters at night to satisfy his awakened appetite for souls, lest he go berserk and lose all chances at finding happiness.