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Internal medicine doctor In-ah believes hormones can take control over 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 nicest 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.