내분비내과 의사 인아는 호르몬이 사람의 건강, 성격, 심지어 사랑의 감정까지 조절할 수 있다고 믿는다. 어느 날, 그의 가설을 감정할 이상적인 연구 대상이 눈앞에 나타난다. 뛰어난 실력으로 유명했던 외과의사 승주는 교통사고로 뇌에 파편이 박히면서 성격이 변한다. 유능한 만큼 경쟁심 강하고 수술의 성공률에만 집착하는 그는 매사 긍정적이고 활기찬 인아를 이해하기 어렵다. 게다가 자신의 운명을 바꾼 그 사고와 관련해 승주는 인아를 오해하고, 인하를 끊임없이 괴롭힌다.
Internal medicine doctor In Ah believes hormones can control a person's health, personality, and even feelings of love. One day, the ideal research subject to test out her theory appears in front of her. That person is surgeon Seung Joo who was once the most excellent person in the world. But after suffering a car accident in which debris becomes lodged in his brain, he turns into an unforgivingly competitive person. After the accident, Seung Joo begins to obsess about only the success rate of his surgeries. The accident left him with abnormally high spatial perception, giving him the ability to simulate surgeries in his mind, and he becomes the star of his department. Meanwhile, after a misunderstanding causes Seung Joo to blame In Ah as the main culprit for having a ticking time bomb stuck in his head, he starts to torment her and ends up forcing her to leave the hospital.