Synopsis
Yoo Si-jin (Song Joong-ki) is the captain of a South Korean Special Forces unit. He and his friend Master Sergeant Seo Dae-young (Jin Goo) are off-duty when they witness a young man stealing a motorcycle and apprehend him. The thief, Kim Gi-bum (Kim Min-seok), is injured during the chase and therefore sent to the hospital. Dae-young realizes his cell phone was stolen by the thief and goes to the hospital along with Si-jin to retrieve it.
In the emergency room, Si-jin meets Dr. Kang Mo-yeon (Song Hye-kyo) for the first time and is instantly attracted to her. Mo-yeon mistakenly assumes Si-jin is part of the thief's criminal gang after hearing him being addressed as "Big Boss", which is actually his call sign. He proves his identity with the help of army surgeon Yoon Myung-ju (Kim Ji-won).
Si-jin and Mo-yeon begin to date, but due to Si-jin being called in for duty, their dates are often interrupted halfway. Si-jin receives his orders to be deployed on a peacekeeping mission in the fictional war-torn country of Uruk. Meanwhile, Mo-yeon is upset because she found out that she had been passed over for professorship in favor of a less-capable colleague who had familial connections to the hospital management. When Si-jin returns and meets Mo-yeon again, she directly asks him to explain his sudden disappearances and his job but he is unable to go into too much detail due to the highly classified nature of his assignments. They talk about their views on life and realize how different they are. Si-jin, as a soldier, kills to protect lives while Mo-yeon, as a doctor, honors the Hippocratic Oath and tries to save lives. Due to their philosophical differences, they agree to part ways. Dae-young, on the other hand, faces a dilemma on whether to continue his relationship with Myung-ju or risk being forced out of the army for breaking fraternization rules by Myung-ju's father, who is a three-star general and commander of the Special Forces.
Eight months later, Mo-yeon rebuffs the romantic advances of the hospital director and, as a result, is assigned against her will to lead a team of medical volunteers to Uruk. There, Si-jin and Mo-yeon reunite and both gain a deeper insight into one another's jobs as their respective teams endure an earthquake and a viral epidemic, while also dealing with a warlord who once fought by Si-jin's side but now takes advantage of the corrupt government of Uruk to make money off human trafficking. While in Uruk, Mo-yeon accidentally confesses her feelings (much to the amusement of their respective teams) and the two officially start dating. She gradually comes to terms with the possibility of him being killed in action and the fact that she may never have full disclosure of his whereabouts. In the meantime, the usually-stoic Dae-young finally works up the nerve to show more affection toward Myung-ju after she nearly dies in quarantine after being infected with the virus.
Si-jin and Mo-yeon continue their relationship after returning to Korea, until Si-jin and Dae-young are sent on a classified operation in which they disappear and are assumed to be dead. Months pass and Mo-yeon mourns Si-jin's death, thus deciding to commemorate his passing by volunteering with a medical team sent to Albania while Myung-ju was sent on a medical mission in Uruk. This is where Si-jin eventually finds her on the day of their one-year death anniversary while Dae-young went to Uruk to find Myung-ju after reporting that they were alive to the Military Headquarters. After their reunion, Si-jin told Mo-yeon that he and Dae-young were rescued from captivity by "a friend from afar", a soldier from North Korea whom Si-jin had helped before. Si-jin and Mo-yeon, along with Myung-ju and Dae-young, are all happily reunited with each other.
