An exceptionally skilled martial artist and swordswoman, Eun-ah is a woman who disguises herself as a man in order to earn a living as hired protection. When one day she by chance saves a young nobleman from being robbed by some crooks, she quickly learns that the young nobleman was actually the Crown Prince of Joseon, and he wants none other than for her to be his personal bodyguard! Trouble is, no one in the palace, least of all the crown prince, knows she's a woman, so she has to keep pretending to be a man lest her true identity is revealed. Things get tricky, however, when the Crown Prince and Eun-ah start developing feelings for each other...