Yeo Hwa gets married into the Second State Councilor's family as the eldest daughter-in-law, the most prestigious family in Joseon. However, she becomes a widow as her husband dies on the day of her wedding. She ends up locked in the house in sorrow for her husband, dreaming of the world outside the gate. Ever since then, she has had a dangerous secret. She jumps from roof to roof at night wearing a mask and looks for people who need help. Her double life was perfect until she met Park Soo Ho, the intelligent yet stuffy officer of the Forbidden Guard Division. His attractive looks, ability, and outstanding swordcraft are second to none. Without knowing whether it is a coincidence or fate, things get complicated as Soo Ho keeps bumping into Yeo Hwa wherever she goes at night wearing the mask. Does this man know about her secret? Yeo Hwa's double life, which she has kept hidden so far, is about to be revealed!