Seoseora, a master of disguise and detective for the Bureau of Royal Investigations, hides her true identity while solving cases. A royal decree declares Crown Prince Lee Seon, infamous for the deaths of his three crown princesses, must take a fourth. Seora meets a carefree man, Nari, who interferes with her work, only to discover he is the crown prince himself. Lee Seon proposes a deal: Seora becomes his fourth crown princess to uncover the murderer behind the previous deaths, promising her anything she desires in return. Reluctantly, Seora accepts and enters a contract marriage. As they work together, Seora finds herself drawn to Lee Seon, whose sincerity and care begin to melt her guarded heart. Their relationship blurs the lines between duty and affection, leaving Seora questioning if their arrangement can remain purely business.