Yoo Si-jin (Song Joong-ki) is the captain of a South Korean Special Forces unit. He catches a motorcycle thief, Kim Gi-beom (Kim Min Seok), with Sergeant Major Seo Dae-young (Jin Goo). The thief is injured during his capture and is sent to the hospital. Dae-young realizes his cellphone was stolen by the thief and goes to the hospital along with Si-jin to retrieve his cellphone. In the emergency room, Si-jin meets Dr. Kang Mo-yeon (Song Hye Kyo) for the first time. He falls in love with her immediately. Mo-yeon mistakenly assumes Si-jin is part of the thief’s criminal gang, due to his nickname “Big Boss”, which is actually his call sign. He proves to her that he is a soldier with the help of army surgeon Yoon Myeong-ju (Kim Ji Won). Si-jin and Mo-yeon begin to date, but due to their jobs, their dates don’t go well. Si-jin receives orders to lead his soldiers on a peacekeeping mission in the fictional country of Urk. Meanwhile, Mo-yeon is upset because she failed to become a professor due to a colleague’s privileged background. When Si-jin and Mo-yeon meet again, they talk about their views on life and realize how different they are. Si-jin, as a soldier, kills to protect lives and Mo-yeon, as a doctor honoring the Hippocratic Oath, tries to save lives. They say their goodbyes. Dae-young, on the other hand, faces a dilemma on whether to continue with his relationship with Myeong-ju or risk being removed from the army by Myeong-ju’s father. Eight months later, Mo-yeon rebuffs the sexual advances of hospital director and as a result is assigned to lead a medical team to Urk. There, Si-jin and Mo-yeon meet and fall in love again. Through their encounters in Urk—an earthquake and an epidemic—their love for each other grows stronger. While in Urk, Mo-yeon accidentally confesses her feelings (much to the amusement of their respective teams) and the two officially start dating. Back in Korea, the two of them continue their dates until Si-jin and Dae-young are sent on an operation in which they disappear and are assumed to be dead. Months pass, and Mo-yeon mourns his death, deciding to commemorate his passing with a volunteer trip to Albania. This is where Si-jin eventually finds her after he and Dae-young are rescued from captivity by “a friend from afar”, a soldier from North Korea whom Si-jin had helped save earlier. Si-jin and Mo-yeon along are happily reunited.