Yi Hwan is a prince with a rare condition that gives his blood a unique blue hue, causing frequent health challenges. Despite his frailty, he is kind. His protector is a 25-year-old swordsman and former assassin, hardened by a life of violence. Initially brusque and irritated by Yi Hwan's presence, the assassin finds himself unexpectedly affected by the prince's resilience and kindness. He grapples with his role as protector, confronting his own humanity.