A useless duke's heir is swapped out for a modern transmigrator who brings modern weapons into the medieval world. Modern Underdog, Medieval King follows Lucien, the heir to a duke whom almost everyone has written off as useless. His life changes when a modern transmigrator takes his place, arriving with a time-space bracelet that links two worlds. The newcomer does not arrive empty-handed. He carries modern weapons into a medieval household that has no idea what to make of them, and the people who once ignored Lucien are suddenly paying close attention. The story turns on the gap between the man everyone expected and the one now standing in his place. Lucien's old reputation makes every move the newcomer makes look like a surprise, and every surprise raises the risk of being found out. The cover lists sci-fi, historical, drama, and billionaire as genres, so the clash between eras runs through nearly every scene. As the weapons spread beyond private conversations, the stakes move from personal confusion to a question of power, where one misstep could cost far more than a reputation. The tone rests on the friction between modern confidence and medieval rank, and the series keeps both sides taken seriously. If you are searching for a body swap with a sharp modern edge, this one delivers it, alongside a time travel premise that never feels like a gimmick. The underdog comeback is central, but the court is never simply foolish, which keeps the power struggle honest. Lucien's position is fragile, and the people around him are watching. How long can everyone keep underestimating him, and what happens when someone finally looks closer?