Based on a popular webtoon, "Moon in the Day" tells a chilling and romantic story that moves back and forth between the past and the present. Han Jun-oh is a top star actor in Korea who is haunted by an inferiority complex, while Kang Young-hwa is a dedicated firefighter who becomes a bodyguard after surviving a fateful accident. When Jun-oh undergoes a near-death experience during a commercial shoot, his body is possessed by Do-ha, a noble elite from the Silla Dynasty with a deep resentment. Do-ha was killed by his beloved wife, Han Ri-ta, and his soul has been trapped in time for 1,500 years, unable to move on. As Do-ha takes over Jun-oh's body to exact his revenge on Ri-ta's reincarnation—who happens to look strikingly like Young-hwa—a complicated web of past grievances, protective instincts, and undeniable romantic tension unravels. The series explores the tragic fate of the past and the dangerous pursuit of closure in the present.