Ha-yeon goes to Upo High School for extra classes. After sharing a special moment with Yu-gyeom, she gets angry at him for already having a girlfriend. However, Yu-gyeom continues to ask Ha-yeon to keep seeing him. The two share another special moment. When Yu-gyeom’s girlfriend, Sae-ryeong, finds out, she visits Ha-yeon late at night.

After a sudden accident, Ha-yeon leaves for Seoul, leaving behind a farewell letter. The police visit her parents and Yu-gyeom, while rumors spread at school that Sae-ryeong has died. After discovering Ha-yeon's letter, her older brother Jeong-yeon heads to Seoul to find his sister, who has become entangled with a human trafficking ring.

Ha-yeon and Yu-gyeom return to their normal lives. Sae-ryeong, who was seriously injured, also returns to school, but a group of students begins to grow suspicious of her identity. When they discover that Sae-ryeong and Yu-gyeom have broken up, they start digging into her past. Despite Sae-ryeong's best efforts to keep things under control, her long-kept lie is eventually exposed.

Sae-ryeong’s lie, which has caused an uproar at Upo High School, reaches Yu-gyeom as well. After running away from school, Sae-ryeong happens to visit Noryangjin, where she encounters Du-hee, Yu-gyeom’s second older sister, who left home years ago. Upon learning this, Yu-gyeom and Ha-yeon head to Seoul to find Sae-ryeong.

After her true identity is fully exposed, Sae-ryeong returns to school, only to face blatant bullying from her classmates. Ha-yeon, who had been intimidated by the relationship between Yu-gyeom and Sae-ryeong, gathers the courage to approach Yu-gyeom. However, feeling guilty as the bullying against Sae-ryeong grows increasingly severe, Yu-gyeom ends up taking her hand when she finds herself in danger.