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.

Jin Hee tracks down her ne'er-do-well sister, Jin Ae, at a club but fails to bring her home. Ah Reum expresses her interest in Chang Min while fellow intern, Im Yong Gyu, tries to show Ah Reum that he likes her. Meanwhile, Chang Min begins to admire Jin Hee's skill and dedication as a doctor.

Chang Min spots Chief Gook and Jin Hee returning to the hospital together after their emergency house call and becomes increasingly jealous over their closeness. While looking for Jin Ae, Jin Hee lets her brother-in-law, Kim Kwang Soo, and nephew stay at her home. The interns celebrate the director's decision to let Jin Hee continue her internship, a decision that cost Chang Min a favor to his uncle.

After being unwittingly left on Chief Gook's bed in the male sleeping quarters of the hospital by Chang Min, Jin Hee berates herself for getting drunk again and is embarrassed to face Chief Gook. Jin Ae reunites with her family after her son contracts meningitis and is brought to the emergency ward.

In the aftermath of their argument, Jin Hee and Chang Min reminisce about the first time they met and how they fell in love with each other. As they leave the hospital after their shift, Chang Min's distracted driving causes Jin Hee to fracture her collarbone.

Chang Min becomes determined to save a patient who is in need of a kidney transplant as Jin Hee tries to convince the patient's soon-to-be ex-wife, who is a compatible donor, to visit him. Ah Reum rejects Yong Gyu's proposal, offering him her friendship instead. Chang Min is invited into Jin Hee's home by her sister and brother-in-law.

As Jin Ae and Kwang Soo move more of their belongings to Jin Hee's place, Chang Min convinces Jin Hee to rest at his place for the next 24 hours while he takes over her shift on his day off. Chief Gook notices an improvement in Chang Min's dealing with patients as Chang Min's mother maneuvers him into a dinner with her and Ah Reum.

Despite Chang Min's pleading for her to stay, Jin Hee refuses and returns home as Chang Min admits to his mother that he would like to start anew with Jin Hee. At home, Jin Hee recalls the final days of her marriage while Chang Min reminisces about the time he proposed to Jin Hee.

Jin Hee and Chang Min are reunited with the priest who officiated their marriage when he turns out to be the guardian of a boy they were treating. Prompted by Jin Hee's disclosure about being a divorcee to the other interns and her earlier outburst at him, Chang Min finally realizes the pain Jin Hee went through during their marriage. Meanwhile, Ah Reum resigns to the fact that Chang Min has no interest in her and Yong Gyu makes a near-fatal error when he prescribed the wrong medication to a patient.

Jin Hee's mother and Chang Min's mother meet each other for the first time after their children's divorce and cause a scene in Chief Gook's office. Chang Min distances himself from Jin Hee as his father's health takes a turn for the worse.

Chang Min's father is hospitalized as his family and Jin Hee wait by his side. Meanwhile, the news about Chang Min and Jin Hee being a divorced couple spread through the hospital staff.

Chang Min's family mourns after his father passes away as Jin Hee and her mother reconcile with Chang Min's mother. Chief Gook is suspended for two weeks after physically clashing with another doctor. Jin Hee rushes to Chang Min's side after he falls ill.