Set in the late Goryeo Dynasty, the film centers on a love triangle with high political stakes. The King (Joo Jin-mo) is a ruler who was raised as a child of the palace, married to a Queen (Song Ji-hyo) he cannot physically be with. Instead, he has a secret, decades-long romantic and physical relationship with his loyal general and bodyguard, Hong Rim (Jo In-sung). When the King is pressured to produce an heir, he asks Hong Rim to sleep with the Queen in his place. What begins as a duty leads to forbidden emotions, betrayal, and a bloody rebellion.