Main cast

• Song Joong-ki as Captain Yoo Si-jin (aka Big Boss)

• Song Hye-kyo as Doctor Kang Mo-yeon (aka Beauty) 

• Kim Ji-won as First Lieutenant Yoon Myung-ju

• Jin Goo as Sergeant Major Seo Dae-young (aka Wolf)

Alpha Team Members

• Park Hoon as Sergeant First Class Choi Woo-geun (aka Snoopy)
• Choi Woong as Staff Sergeant Gong Cheol-ho (aka Harry Potter)
• Ahn Bo-hyun as Sergeant First Class Im Gwang-nam (aka Piccolo)

Haesung Hospital Medical Services Team

• Onew as Lee Chi-hoon (1st year resident of Thoracic Surgery)
• Lee Seung-joon as Song Sang-hyun (General Surgeon)
• Seo Jeong-yeon as Ha Ja-ae (ER Nursing Team)
• Park Hwan-hee as Choi Min-ji (ER Nurse)

People at Haesung Hospital

• Hyun Jyu-ni as Pyo Ji-soo (Pathology Specialist)
• Tae In-ho as Han Suk-won (Chairman of Haesung Hospital)
• Park Ah-in as Kim Eun-ji (Thoracic Surgery Specialist)
• Jo Woo-ri as Jang Hee-eun (1st Year Resident of Anesthesiology)

Tae Baek Army

• Kang Shin-il as Lieutenant General Yoon Gil Joon (aka Yellow Tiger) (Myung-ju's father)
• Kim Byeong-cheol as Lieutenant Colonel (later promoted to Colonel) Park Byeong-su
• Kim Min-seok as Lance Corporal Kim Gi-beom (later promoted to Platoon Sergeant as seen in EP 16).^[6] A pickpocket later enlisted to the army.

Urk area

• Jasper Cho Tae Kwan as Daniel Spencer (Peacemaker Emergency Doctor Team)
• Jeon Soo-jin as Ri Ye-hwa (Peacemaker Emergency Nurse Team)
• David Lee McInnis as David Argus (Gang Boss, former Delta Force Captain), the main antagonist.
• Dean Dawson as the Chief of Police (Tae Baek Police Chief)
• Jo Jae-yoon as Jin Young-soo (Chief Manager of Uruk Electric Power Corporation)
• Lee Yi-kyung as Kang Min-jae
• Zyon Barreto as Fatima
• Elena Zhernovaia as Valentine
• David Pipes as Martin (World Times reporter, episode 7)

Others

• Ji Seung-hyun as Senior Lieutenant Ahn Jung-joon, Korean People's Army.
• Kwak In-joon as Lee Han-soo (Minister of Foreign Affairs)
• Joey Albright as US Army Delta Force member

Cameos

• Lee Kwang-soo as shooting arcade operator (episode 1)^[7]
• Lee Jong-hyuk as Captain Kim Jin-seok (episode 2, 5, 10, 15)
• Matthew Douma as Jordan, US Army Delta Force Captain (episode 2, 10, 12)^[8]
• Jung Ji-won as TV announcer (episode 2)
• Park Tae-won as TV announcer (episode 2)
• Ryu Hwa-young as Seo Dae-young's ex-girlfriend (episode 4)
• Jun In-taek as Yoo Young-geun (Yoo Shi Jin's father) (episode 6, 7, 15, 16)
• Park Joon-geum as Lee Chi-hoon's mother (episode 7, 11)
• Yoo Ah-in as bank reception Eom Hong-shik (episode 13)
• Nam Ki-ae as Kang Mo-yeon's mother (episode 13)
• Red Velvet as themselves (episode 16)

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 Myung-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 Uruk. Meanwhile, Mo-yeon is upset because she failed to become a professor because of 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 Myung-ju or risk being removed from the army by Myung-ju's father, Lieutenant General Yoon Gil-joon (Kang Shin-il).

Eight months later, Mo-yeon rebuffs the sexual advances of hospital director Han Suk-won (Tae In-ho) and as a result is assigned to lead a medical team to Uruk. There, Si-jin and Mo-yeon meet and fall in love again. Through their encounters in Uruk—an earthquake and an epidemic—their love for each other grows stronger. While in Uruk, Mo-yeon accidentally confesses her feelings and the two officially start dating, much to the amusement of their respective teams.

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 earlier helped save the life of. Si-jin and Mo-yeon are happily reunited. Meanwhile Dae-young goes back in an emotional reunion to Myung-ju, who is stationed in Uruk.

In the credits, the entire team is once again called into action following a volcanic eruption in Canada.

Here is a video clip of this series, enjoy watching.