Kim Hae-Kyung (Song Seung-Heon) is a successful psychiatrist, but he has only experienced heartbreak with women in his personal life. He is also a foodie and sometimes meets his clients, while having a meal.

Woo Do-Hee (Seo Ji-Hye) works for a media company that produces content for streaming services like YouTube. She goes to Jeju with the feeling that her boyfriend will propose to her there, but she only finds heartbreak. While she reels from her latest breakup, she runs into Psychiatrist Kim Hae-Kyung, who sat next to her on the flight to Jeju, and they decide to have dinner together. A love-hate relationship soon develops between these two headstrong individuals, even though they don't know each other's name.

When they get back to Seoul, Woo Do-Hee's boss wants her to produce a new show with Psychiatrist Kim Hae-Kyung or she will have to work with her ex-boyfriend. Woo Do-Hee is unaware that Psychiatrist Kim Hae-Kyung is the man she sat next to on the flight to Jeju. They meet again accidentally one evening and decide to become dinner mates.

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Shi-Jin (Song Joong-Ki) is the captain of the special forces. He catches a motorcycle thief with Sergeant Major Dae-Young (Jin Goo). The thief is injured during his capture and is sent to the hospital. Dae-Young realises his cellphone was stolen by the thief and goes to the hospital to retrieve his cellphone.

In the emergency room, Shi-Jin meets Dr. Mo-Yeon (Song Hye-Kyo) for the first time. He falls in love with her immediately. Mo-Yeon mistakenly assumes Shi-Jin is part of thief's criminal gang. He proves to her that he is a soldier with the help of army doctor Myeong-Joo (Kim Ji-Won).

Shi-Jin and Mo-Yeon begin to date, but due to their jobs their dates don't go well. Shi-Jin takes an order to lead his soldiers on a peacekeeping mission to the country of Uruk. Meanwhile, Mo-Yeon becomes upset that she fails to become a professor due to a colleague's privileged background. When Shi-Jin and Mo-Yeon meet again, they talk about their views on life and realise how different they are. Shi-Jin, as a soldier, kills to protect lives and Mo-Yeon, as a doctor, tries to save all lives. They say their goodbyes.

Eight months later, Mo-Yeon rebuffs the sexual advances of hospital chairman Suk-Won (Tae In-Ho) and as retribution is assigned to lead a medical team in Uruk. There, Shi-Jin and Mo-Yeon meet again.

Ref: http://asianwiki.com/Descendants_of_the_Sun