Someone who kills and someone who saves lives- this keeps the main characters of the story to have a happy ending.

Yoo Si-jin, played by Song Joong-ki, is a captain of a Special Force Unit while Dr. Kang Mo-yeon, played by Song Hye-kyo, is a doctor. Their first meeting is a disaster as Dr. Mo-yeon thought that Si-jin is a member of a gang because of its call sign "Big Boss" not knowing that the latter is a captain.

From that moment, the connection between the two had been so strong. Si-jin has been attracted to the doctor from the very start from they are being kept away from each other because of some circumstances.

Every goodbye of the two is the heartbreak of every viewer. This is proven by having almost 40% of peak-rating and rated as the Most Popular Show of the year by Korea Broadcasting Advertising Corporation.

Another love story of Si-jin's best friend Sergeant Seo Dae-young portrayed by Jin Goo and army surgeon Yoon Myung-ju played by Kim Ji-won spiced up the story. The viewers found the story of the two an extraordinary one. Their story is not so far from the story of the main characters- loving each other but always been separated because of Myung-ju father being against their love story.

The story had been in its climax when the two main characters finally been together but Si-jin had a tragic accident and assumed to be dead. After a year, he had found to be alive and the two had a happily ever after. Dae-young and Myong-ju have also been happily reunited together same with Si-jin and Mo-yeon.

Because of a great story, the Kdrama had aired for 16 episodes, from February 24 to April 16, 2016, and additional 3 special episodes.

The success of this series also been proved by the rise of the tourism of South Korea as it has been credited to be one of the reasons. Another proof of its success is that it was aired in one of the most secluded countries in the world; North Korea.

It was truly one of the most successful Kdrama series ever made.