Choi Jin-sang (1st year of residency) / actor Cha Tae-hyun
His mother who works at a ranch wishes that he’ll become a professor at medical school but during his six years of medical school and first year as an intern, he constantly got into trouble and his grades were at the bottom of the class. When he joins the residency, his fellow colleagues are Hyun-woo, who was always at the top of his class, and Ha-yoon, who passed the bar examination. At the outset, he faces a tough situation. He lacks in every way as a doctor but he’ll go to great lengths to save a patient’s life.

Jung Ha-yoon (1st year of residency) / actress Kim Jung-eun
After passing the bar examination upon graduating from law school, she decides to join the residency program at a general hospital as a way to prepare herself to become a lawyer who specializes in malpractice cases. With memories of her dad who died from a botched operation, she is compassionate towards patients and has a critical view towards doctors who compromise their values. But while working with her colleagues, she gradually becomes a doctor in her own way and her views also change.

Baek Hyun-woo (1st year of residency) / actor Ryu Jin
In medical school, he was the top student and Ki-tae recruits him to join the medical research team. He’s an impassionate thinker but he does have a good heart. When Ha-yoon needs help in finding a kidnapper, he does his best to track down the criminal. He develops feelings for her as he helps her in every way he can.

Kang Eun-ji (1st year of residency) / actress Koh Joon-hee
She helped Jin-sang whenever he got in trouble during medical school. During exams, she would share her notes with Jin-sang. But Jin-sang is not aware that she has a crush on him. Though she is glad that Jin-sang is showing himself to be a promising doctor, sshe starts to get worried when Jin-sang and Ha-yoon keep getting into petty arguments...

Kim Do-hoon (Staff doctor) / actor Lee Jae-ryong
After working for just one year in his residency, he is asked to join the medical staff as a staff doctor. He was once a handful but now he mentors younger doctors. He has a warm heart and one time, he left in the middle of a demonstration for a $20 million state-of-the-art medical device to operate on a patient in the emergency room. He’s a doctor who is dedicated to his work of saving lives and that means he is not so keen on advancing his career.

Han Ki-tae (Staff doctor) / actor Lee Jong-won
Having the right connections, he’s a star doctor at Sungui University Hospital who is admired by his peers for his skills. Although he is one of the top surgeons in Korea, he selectively chooses the patients he’ll operate on to keep his career unblemished. He becomes the head of a medical research team that is supported by the hospital and becomes adversaries with Do-hoon who has different principles.

Song Hye-soo (ER department staff) / actress Doh Ji-won
As a senior doctor at the ER, where there’s always an emergency happening, she is a skilled surgeon who makes quick calculated decisions. She is a perfectionist at work but also has a softer side that she reveals when she’s around Do-hoon.