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 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 thief's criminal gang. He proves to her that he is a soldier with the help of army doctor Yoon Myeong-joo (Kim Ji-won).
Si-jin and Mo-yeon begin to date, but due to their jobs, their dates don't go well. Si-jin takes an order to lead his soldiers on a peacekeeping mission to the fictional country of Uruk. Meanwhile, Mo-yeon becomes upset that she fails 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 realise how different they are. Si-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 Han Suk-won (Tae In-ho) and as a result is assigned to lead a medical team in Urk. There, Si-jin and Mo-yeon meet again and fall in love again. Through their encounters in Urk, such as an earthquake and an epidemic, their love for each other grows stronger. While in Urk, Mo-yeon accidentally confesses her feelings and the two officially start dating.
Back in Korea, the two of them continue their dates until Si-jin is sent on an operation when he disappears and is assumed to be dead.