The narrative picks up with the hero Gelm, warrior Zott, mage Meralda, priestess Mari, and thief Keith arriving at the Demon Lord's throne room. The Demon Lord, an old man, acknowledges their arrival, noting the effectiveness of their scout, Keith, in defeating his Four Heavenly Kings individually. Gelm, however, dismisses this, attributing their success to their own strength and challenging the Demon Lord.
A fierce battle ensues. Mari's shield magic deflects the Demon Lord's shockwaves, Zott's axe shatters ice blades, and Meralda's flames weaken him. As Gelm prepares to strike, Keith, observing the fight from the rear, realizes Gelm's timing is too early, leaving him vulnerable to a devastating shockwave. Acting on instinct, Keith sprints forward, surpassing Gelm's speed. Using his skill, "Swift Thief," Keith snatches a red gem from the Demon Lord's mantle, which appears to be a source of his power. With the gem gone, the Demon Lord's attack falters, allowing Gelm to land a fatal blow with his legendary sword, Flamnagnum. The Demon Lord dies, warning of future successors.
Gelmu, dismissive of Keith's crucial action, snatches the gem, calling Keith's thievery "dirty." Mari, however, recognizes Keith's contribution, assuring him that his efforts will be appreciated someday. Zott, possessive of Mari, grows impatient with their conversation. The party then loots the castle, finding little of value. During the treasure distribution, Mari protests the unfairness, advocating for rewards based on contribution. Meralda scoffs, suggesting Mari share her own spoils. Mari generously gives Keith the Demon Lord's mantle, acknowledging his vital role.
The party arrives at Elem town to a hero's welcome and a celebratory feast. Keith, feeling ostracized and weary of the journey, retreats to his inn room. He reflects on his participation in the quest: to secure the release of the Aldelberg Thieves' Guild, his adoptive family, who were promised freedom by the king upon the Demon Lord's defeat. Despite the hope of their liberation, Keith is filled with resentment and exhaustion.
Later, Gelm, Zott, and Meralda return to the inn, drunk. The innkeeper delivers a letter from the king, specifically instructing it be given to them in Keith's absence. Upon reading it, the trio's demeanor shifts to cold amusement. They then confront Keith in his room, revealing the king's order: Keith, as a thief, is inconvenient to the party, and they are to eliminate him. Keith realizes the king never intended to release the guild and has been betrayed. Gelm attacks, and a desperate fight for survival begins. Keith uses his skills to evade and counter, but is severely wounded by Zott and Meralda. He manages to escape the inn, pursued by Meralda's flames, and collapses in an alley.
Mari finds Keith and heals his wounds. Keith, weak but resolute, implores Mari to keep his survival and her healing a secret, urging her to return to the party and cover for him. He then dons the Demon Lord's mantle, which inexplicably fastens itself, and loses consciousness.
Keith awakens in a vast, dark hall, facing the Demon Lord, who is inexplicably alive, his sword still impaled in his chest. The Demon Lord reveals his immortality, inheriting power from past demons. He taunts Keith, calling him a thief who has always run from his problems. As the Demon Lord speaks, a pool of black blood spreads, and Keith's shadow transforms. He realizes he is sinking into a bottomless pit.
Keith finds himself in a hall with four giant crystals, containing the frozen forms of the Four Heavenly Kings. He despairs, believing he has died and gone to hell, his efforts to save his family futile. In his rage, he screams at the heavens, demanding an answer for his fate. He then notices a mirror, the "Mirror of Identification." Upon looking into it, he sees two horns on his head and his profession displayed as "Phantom Thief Demon Lord," a supreme class capable of dominating demonic kin and stealing all he sees. He laughs at the absurdity of his transformation.
His laughter shatters the crystals, reviving the Four Heavenly Kings: Summoner Diana, Saber Man Ginrou, Gunslinger Viktor, and Dark Mage Allelaquolier. They recognize Keith as their new master, the Demon Lord. Keith, overwhelmed, accepts his new role and commands them to stand. He learns they were not melted down as failures but revived by him, a testament to his power. He lies about his intentions, claiming he gave them a second chance, which moves them to tears.
Keith, still weak from blood loss, requests food. The Four Heavenly Kings offer him a grotesque feast, but he insists on eating with them. They are surprised by his humility, mistaking his need for blood as a desire for shared hardship. They agree to eat with him, and Joseph, a butler, arranges a lavish meal. During the meal, Diana notices the Demon Lord's mantle is missing its gem, stolen by the hero. She explains the gem's importance for the castle and the Demon Lord's power. Keith realizes his current strength is only half of the previous Demon Lord's, but Diana assures him his "Phantom Thief" class offers immense potential.
They visit the Demon Lord's armory. Keith uses his "Appraisal Eye" skill to examine items. He finds the "Monocle of Conviction," which allows him to read others' stats and skills. He tests it on Diana, confirming his ability to steal skills. He then uses this to "steal" the "Summon Magic" skill from Diana, allowing him to summon a demon beast. He returns the skill, and Diana, realizing his intent, summons a powerful beast.
Keith, accompanied by Diana and the revived Aldelberg Thieves' Guild, uses a gate to infiltrate the kingdom's prison. He incapacitates guards and frees his family, who are overjoyed and amazed by his transformation. He then returns to the Demon Lord's castle, leaving his family to escape, with a mission to confront the king.
Keith, disguised as a guard, infiltrates the castle and overhears the king and a general discussing his supposed death and the betrayal of the Aldelberg Thieves' Guild. He reveals himself as the "Phantom Thief Demon Lord," stealing their swords and locking them in the room. He then proceeds to the royal palace, where he confronts the king and general, using his skills to intimidate them.
Back at the castle, Keith reunites with Diana and the Thieves' Guild. They ride giant wolves, and Keith rides with Diana. They establish a luxurious temporary residence. Keith learns from Diana that the castle's rooms are inaccessible without the Demon Lord's magic stone, which was stolen. He also learns that his new class, "Phantom Thief," combined with his former human experience, makes him a formidable Demon Lord.
Keith, now a Demon Lord, declares his intention to free the Aldelberg Thieves' Guild. He selects Diana and Allelaquolier for the mission, emphasizing the need for stealth and non-violence. Using Allelaquolier's magic and his own skills, they infiltrate the prison and free the guild. Keith then returns to the castle, intending to confront the king.
Keith, using his disguise and infiltration skills, navigates the castle and overhears the king and a general discussing the successful elimination of the thief Keith and the betrayal of the Aldelberg Thieves' Guild. He reveals himself as the "Phantom Thief Demon Lord," stealing their swords and locking them in the room. He then proceeds to the royal palace, where he confronts the king and general, using his skills to intimidate them.
Keith, now a Demon Lord, reunites with Diana and the Aldelberg Thieves' Guild. They ride giant wolves, and Keith rides with Diana. They establish a luxurious temporary residence. Keith learns from Diana that the castle's rooms are inaccessible without the Demon Lord's magic stone, which was stolen. He also learns that his new class, "Phantom Thief," combined with his former human experience, makes him a formidable Demon Lord.
Keith, now a Demon Lord, declares his intention to free the Aldelberg Thieves' Guild. He selects Diana and Allelaquolier for the mission, emphasizing the need for stealth and non-violence. Using Allelaquolier's magic and his own skills, they infiltrate the prison and free the guild. Keith then returns to the castle, intending to confront the king.
