Medical student Oh Chang Min and nutritionist Oh Jin Hee married six years ago despite objections from Chang Min's family but their marriage didn't last long as they began to frustrate and resent each other for the challenges they faced, leading to a divorce. Now, as Chang Min starts work as an intern at a university hospital, he is surprised to find that Jin Hee is also a medical intern at the same hospital.

Reporting late on her first day of work, Jin Hee is scolded by her boss, Gook Cheon Soo, who had saved her life the night before in the same ER department where she is about to start her internship. Though she convinces Chief Gook not to fire her, she is ostracized by the rest of the ER interns and is constantly reminded of her inadequacies as a doctor.

Jin Hee strives to resuscitate Chang Min after inadvertently shocking him with a defibrillator as she tried to stop an out-of-control patient from shooting him with a gun. The other interns are amazed by Jin Hee's tenacity but Chang Min is angered by the fact that she almost killed him. After recovering, Chang Min is sent on a blind date by his mother.

Trapped in an elevator while transferring a patient to the operating theater, Jin Hee and Chang Min are forced to perform a tracheotomy on the patient without supervision. While the other interns are in awe of Chang Min, who they thought had performed the procedure, Chief Gook discovers that Jin Hee was the one who did it. Meanwhile, Chang Min and his mother find out that the blind date who stood him up was fellow intern, Han Ah Reum.

The doctors convene an emergency meeting to investigate the identity of the ER intern who had performed the tracheotomy on a esophageal cancer patient who is now dead and whether the operation contributed to his death. Chang Min's mother is admitted to the hospital as a VIP patient and is surprised to find out that Jin Hee is an intern working alongside her son.

Chang Min's mother demands that Jin Hee be replaced by Han Ah Reum as her doctor. Confronted by animosity from Chang Min's mother and her possible dismissal, Jin Hee decides to resign. However, after seeing Jin Hee taking the initiative to help an outgoing patient to get treatment, Chief Gook rejects her resignation and encourages her to continue her internship.