Yoo Hajin, who works as a private tutor for extra income, has a student: Im Ildo, a wealthy student preparing to retake his college entrance exams. However, one day, she hears shocking news from her younger sister. "Sis, Ildo is... my junior at university!" It turns out that Ildo, whom she thought was a struggling repeat student, had already been accepted into National University. From the day his true identity is revealed, Ildo’s meticulous schemes and terrifying obsession begin to slowly tighten their grip on Hajin's life.