A little confusing (SPOILERS)

(spoilers) The Former King was stepson to the Queen Mother (whom she poisoned a little later in the movie), who manipulated EVERYONE. She told him enough time had gone by since his father's death that he take a wife/concubine to produce an heir. He agreed and asked her to set him up with someone. She then set him up with the main character who just got caught running off with her lover. In order to punish her, the queen mother selected her to be the concubine to the King. The gap made sense to me, because the movie subtitle said 5 years later, and because then you see she is the new 'queen/concubine' and they had produced the desired heir who is shown to be about 4 years old.

Her reaction to the sex scene with the new king threw me as well. Perhaps she was trying to come to terms with her initial plan to kill him and then had her doubts because maybe she sympathized with him since all he did was love her and honor all her wishes. Yet, went through with killing him to protect her son and secure his future.

I think the closing scene with the small boy on the throne shows that her plan worked. The closing of the doors and her looking back over her shoulder in surprise may mean that the 'servants' who were the only ones left, because the Queen Mother took her own life with the sword, covered up the murderous plot and they closed the door as they did before with the doors and windows which signaled to her that she would not be in trouble and that the new king (the boy) is the new true King.

Korean monarchs often had legitimate wives, that serve as traditional queens, and as many 20+ concubines who serve the king in an almost exclusively sexual manner. If the queen dies, the king can choose one of his concubines to assume her role or he can just remain unmarried. Any progeny from the relationship between a king and concubine can be considered legitimate and placed in line for the throne.

The main character was never married and remained a concubine. That's why the dowager queen was the one calling the shots even though The Concubine was the mother of the late king's heir. The Concubine is the daughter of lower ranked government official. She catches the eye of the prince, but his mother disapproves so she gives The Concubine's father a higher position in exchange for his cooperation in setting his daughter up with the king. This is all to prevent the prince from pursuing her.

From there, 5 years pass and The concubine now has a son, yet the king has fallen gravely ill. The Dowager Queen and the Prince now ascend to the throne. The Queen and her eunuch actually poisoned the late king to death because A)she wanted her son on the throne and B) The legitimacy of the king's son was suspect.

The concubine must ensure the safety of her son, and the only way to do that is by killing anyone who stands between him and the throne. That includes the Prince. As for the sex scene, we're shown that despite his paranoid outbursts The Prince is actually quite skilled at hiding the salami. I think part of her reaction is that this is the first time she's genuinely enjoying sex and it's with the least likely person. Another part of it is that she's trying to get control and the Prince is being super aggressive. She also seems to feel a bit conflicted on the finale as well.

I think the final scene was symbolic of her always having to look over her shoulder because she no knows how treacherous court life can really be.

I had a hard time keeping up as well. I just came here to make sure it wasn't just me. I thought I accidentally hit fast forward or something. I didn't read the spoilers so I'm now going back to try and finish it.