In a world where an individual's magical skill determines their social standing and power, Fate Graphite has been relegated to the lowest rung of society as a castle gatekeeper. His only skill, Gluttony, is considered completely useless, as its only apparent effect is to curse him with an insatiable, eternally gnawing hunger that leaves him weak and perpetually starving. He is mocked and lives a miserable existence until one night he kills a thief who has infiltrated the castle. In that moment, the true, terrifying nature of his skill awakens: Gluttony does not just cause hunger, but allows Fate to devour the very skills, strength, and souls of any being he kills, converting them into his own power.

This revelation upends his life, but the power comes with a price. Fate soon acquires a talking, sentient black sword named Greed, a legendary Mortal Sin Weapon that embodies the sin of avarice. Greed becomes his partner, offering snarky guidance and power-enhancing abilities in exchange for a constant supply of souls to feed its own hunger. As Fate absorbs more power, he quickly grows stronger, but he also risks entering a "berserk" state where his Gluttony overtakes his sanity. He is driven not only by a desire to escape his past but also by a quest for revenge and a need to protect those he comes to care for.

Central to this journey is Roxy Heart, a kind and just holy knight from the prestigious Heart family. Possessing the Mortal Sin skill of Wrath, which ignites her fighting spirit, Roxy acts as a moral anchor for Fate, saving him from his initial spiral into darkness and inspiring him to use his powers for good. Their relationship deepens as they face numerous threats across the kingdom, from the fortified city of Babylon to the monster-ridden continent of Galia. They are pursued by powerful antagonists who wield other Sin skills, including the manipulative Myne with the Envy skill, who initially poses as a love interest before betraying Fate, and the arrogant noble Eris Seifort, whose Pride skill makes her a formidable duelist.

The narrative is structured around the lore of the seven Mortal Sin skills, which are conceptual powers born from ancient experiments that grant incredible strength but also impose heavy burdens. The world is dominated by a hierarchy of Paladins, elite warriors who wield holy swords, with noble houses like the Barbatos and Brerick families clashing over power and legacy. As Fate travels, he learns that the Barbatos family, his own lineage, holds secrets tied directly to the nature of Gluttony. The first major narrative arc follows Fate as he masters his initial powers with the help of Greed, forges his bond with Roxy, and confronts a sinister conspiracy involving human experimentation led by the House of Brerick. His journey culminates in a desperate mission to rescue Roxy, who faces a death sentence in the dangerous lands of Galia, forcing Fate to push his Gluttony to its absolute limits against overwhelming demonic and draconic foes. Through these trials, the boy once ridiculed as talentless begins to climb from the depths, but the constant struggle to balance his growth with the corrupting toll of his curse remains the central conflict of his path.