[Spoiler] "Scarlet Heart: Ryeo" IU finds out about the spy and rages at Lee Joon-gi

On the eighteenth episode of the SBS drama "Scarlet Heart: Ryeo", Hae-soo (IU) was betrayed by Chae-ryeong (Jin Ki-joo).

Wang So (Lee Joon-gi) who was ascended to the fourth king, found out that Chae-ryeong had become a spy to hurt Hae-soo. He beat her to death knowing she was acting under Wang Won (Yoon Sun-woo) and Wang Wook's orders.

Hae-soo was furious. "How can you beat someone to death?!"

Wang So explained, "Chae-ryeong has been spying on you. She wanted to tell the ninth Prince about us so that it would go into the ears of the empress and erase any traces of you, making you a traitor. She's responsible for our marriage being stopped. It was also Chae-ryeong that used mercury on my brother".

He added that Wang Eun (Baekhyun) wasn't able to run because of Chae-ryeong's spy act as she helped Wang Won and Wang Wook let out their men to hunt him.

Hae-soo was shocked but she said, "Her only crime is to have been born as a slave. She was a sister of mine".

Hae-soo later received a letter from Chae-ryeong saying she had loved Wang Won and she'd become a spy for him.