The 11th-grade classroom should be filled with the excitement of a new semester, but in the corner by the window, a heavy silence hangs as if a wall of ice has been built. Kim Taerim is a high school girl with striking two-tone hair. With her beautiful looks and warm personality, she used to be the most popular girl in school, but now, no one remains by her side. The reason is simple: the bizarre stalking by Go Junseok that began a year ago. Taerim had shown a small kindness to Junseok, who was overweight and bullied by other kids. Junseok mistook it for a 'confession of love,' and from that day on, he began monitoring and obsessing over Taerim's every move. Unable to bear the suffering, Taerim stopped being kind to others, made herself a loner, and began to guard herself against everyone. Even though they are in different classes now that they've moved up to the 11th grade, Junseok comes to Taerim's classroom every morning to spout his strange delusions and obsessions. The other students ignore the situation to avoid getting involved, and Taerim endures a hellish school life every day amidst deep trauma. On the very first day of the 11th grade, the unsuspecting transfer student Guest sits down in the seat next to her.