Long ago, a king forged a pact with the demon lord Maou, obtaining a stone known as the Demon King's Stone. In exchange for his wish, the contract unleashed monstrous beings called Crows upon the world, an event remembered as the First World Decadence. After the king's death, the stone shattered into thirteen pieces and scattered across the globe, each fragment still capable of summoning Crows and granting immense power to its holder. As a result, thieves and treasure hunters everywhere began searching for the fragments, hoping to reunite them and claim a wish.

In a bustling bazaar, a carefree, curry-obsessed young man named John Elegadine encounters a skilled lock-picking thief named Lu Larue. John claims to be a thief himself, though his talents are limited to brute force rather than finesse. He carries a secret: the soul of the demon lord Maou is sealed within his right arm, granting him devastating destructive power. When he unleashes this force, his hair turns silver, and silver feathers scatter around him, earning him the nickname "the Silvery Crow." However, using this power carries a grave risk—overexertion threatens to transform him into a Crow completely.

Lu, who is actually a descendant of the royal bloodline that originally contracted with Maou, decides to partner with John. Together they set out to gather all thirteen Demon King's Stone fragments. Their journey brings them into conflict with the Great Thief Band "Red Wind" led by the volatile and powerful Kazami. Kazami proves both a rival and a temporary ally as they clash over stones and cross paths repeatedly. Along the way, John's past is revealed: he was raised by a thief named Kuroe, who was killed by the villainous Fukurou (the Owl), a man who orchestrated the Second World Decadence sixteen years earlier by collecting all the stones and nearly reviving Maou. It was only by sealing Maou's soul into the infant John's arm that the disaster was averted.

The pair travels to the floating mechanical city of Centraleague, a gleaming utopia powered by a Demon King's Stone. Beneath its clean surface, however, the city hides a dark secret: the sick, elderly, and disabled are exiled to the underground as "Second-Class Citizens." There they meet the "Mouse Thief" Michelob Albatross, a kind-hearted boy who steals gold coins and leaves them for the oppressed. After battling Fukurou's agents and gaining three powerful stone fragments (the fangs, blood, and scales of Maou), John and Lu confront Fukurou directly. But the true mastermind behind the plot is revealed to be none other than Kuroe himself—John's foster father, who faked his death and turned into a ruthless enemy determined to revive Maou and reshape the world.

In the final confrontation, the thirteen stones are united, and Lu performs a forbidden contract that allows Maou to manifest physically. John must use his own body as a vessel to contain the demon lord's soul once more, ultimately sacrificing his right arm to seal Maou away again. The world is saved, but the cost is high. John and Lu part ways, each continuing their own path—John wandering as a thief with a newfound purpose, and Lu embracing her royal heritage. The legacy of the Demon King's Stone lingers, but for now, the threat of total destruction is averted.