"Great Doctor" Kim Hee Sun was poisoned by Park Yoon Jae.

On the Tuesday episode of 'Great Doctor,' Duk Heung (Park Yoon Jae) finally reveals his true motives towards Yoo Eun Soo and that he had been secretly and slowly poisoning her. 

Choi Young told Duk Heung to take the notebook and show it to Yoo Eun Soo and it seemed that Duk Heung was agreeing with him easily. She copied the numbers on the notebook on another piece of paper and she realized that it was the exact time that the sky portal would open. However, she does not tell him this. After acquiring the contents of the notebook, Choi Young and Yoo Eun Soo try to find the portal and leave the palace at night. Ki Chul, who finds out about this, becomes furious but Duk Heung finds Palace Servant Choi, who is able to communicate with Choi Young. 

Duk Heung tells Palace Servant Choi that he gave Eun Soo paper that when the user touches it, they have to get a bit of saliva on their fingers because it's so thin. So the users unknowingly become poisoned by the paper. He says, "The poison is incredibly strong and the symptoms are not instant. It builds and hits the victim suddenly." He reveals that he has the antidote, which gives the servant no choice but to find them. 

At the same time, Yoo Eun Soo already started to complain of pain and Choi Young panics because he does not know what to do about her suffering.