After I mentioned before, some critics say it is a remake of Eunuch, 1968 by SHIN Sang-ok. I agree with it because structures, plots, characters are adopted from Eunuch.
The Concubine has many plots but they are well weaved, which means, it is directed wonderfully. Mise-en-scene is also really good, which makes it even classy.
Prince Seong-won falls in love with Hwa-yeon. Queen, who wants Seong-won to be a next king, notices it ordering Hwa-yeon to be a concubine, his brother, king's wife.

On the other hand, Kwon-yoo and Hwa-yeon love each other. They decide to run away but the elopement fails. She pleads with her father not to kill Kwon-yoo in condition with she goes to the palace. Her father castrates him instead. Later Kwon-woo becomes an eunuch to revenge on her father.
Queen plans to put his son on the throne with a minister. She poisons king and eliminates her opponents including Hwa-yeon's father. The regency by the queen mother begins. Hwa-yeon is isolated with a young child.
Queen wants King Seong-won to have a son to consolidate the power. The sex should be followed by instructors next to the doors.

However, In the king's mind, there is only one love, Hye-yeon. He takes her maid imagining her.
Hwa-yeon uses king saying she can be his wife when he becomes a true King. She is wearing a hairpin which was presented before.
Hwa-yeon makes up that queen tries to poison the king. King is furious. Eunuch Kwon-yoo helps her because the young child are theirs.
While making love she kills king with the hairpin.
Please watch director KIM Dae-sung's great movie, Bungee Jumping of their Own, 2000.