A thousand years ago, during the Unified Silla period, people feared a gigantic sea monster known as the Dragon King and offered human sacrifices to appease it. Jang Bo-go, a legendary Silla general, is secretly dispatched to China as a spy but is forced to flee when his identity is exposed. To return home, he must take the forbidden sea route where the Dragon King is said to appear. When the monster finally emerges, Jang Bo-go and his crew face a force far beyond any human army. The Sea Serpent is a Korean maritime fantasy adventure that combines historical legend, survival action, and creature spectacle, with strong potential for Asian markets and international film rights sales.