The Secret Night Tutor.

Hong Dan, a court lady assigned to teach Crown Prince Heon the arts of the night, is hiding a secret—she must never be discovered as a virgin. Amid suspicious rumors, their perilous lessons continue, and the two begin to uncover each other's true feelings. But sensing their growing closeness, Crown Princess Seryeong starts to tighten her sharp and relentless pressure. In the end, through Seryeong's scheme, Dan's true identity is exposed, and she is expelled from the palace...