Viewers are extremely curious about which side Kim Jae Joong will take at the end.

On episode 18 of 'Dr. Jin', which aired on July 22, Kim Kyung Tak (Kim Jae Joong) becomes a double agent at the request of his father and deceives Lee Ha Eung (Lee Bum Soo) and his friend Hong Young Hwee.

Kim Kyung Tak secretly infiltrates Lee Ha Eung's room and searches his desk for a letter that was sent by Jin Hyuk (Song Seung Hun). This is because the letter explains the future of Chosun.

Kyung Tak learns through the letter that Lee Ha Eung's wife is Catholic. He confronts Jin Hyuk asking who he really was. Kyung Tak, however, does not give the letter to his father. Instead, he only partially reveals what he has discovered to his father. Kyung Tak is not completely on his father's side.

The ending of the drama will depend on which side Kyung Tak chooses, his father or Lee Ha Eung and Hong Young Hee. It is also a mystery as to how Kyung Tak will act towards Hong Young Rae (Park Min Young) now that he knows of her Catholic faith.