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Ha-yeon, an ordinary second-year high school student, heads to Upo-ri with her parents for summer vacation. Struggling with friendships and family relationships, she unexpectedly meets Yu-gyeom, the young heir of a traditional Korean family. The two then experience something unbelievably mysterious.

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.