Ten years ago, teenage Choi Jung-woo (Kwak Dong-yeon) is the new student at Namil High School.

After transferring schools seven times because of his father's job, Jung-woo has learned to live like an "invisible man" and is determined not to make friends.

But Yang Ah-young (Lee Se-young), the smart and seemingly aloof Class President, is actually full of empathy, so she nags him into doing many school chores, which annoys Jung-woo.

Meanwhile, his shy seatmate Im Deok-won (Kwak Jung-wook) is being bullied by senior student Shin Young-bok (Park Jung-min) into doing errands for him.

To his shock, Ah-young accepts his offer instead, making their classmates overjoyed and most people at Namil High, including the senior bullies, going ballistic about this new "class couple."

Though unbelieving in the paranormal, Ah-young is scared by the thought of seeing ghosts and enlists Jung-woo's help in illegally breaking into the school at night to get her "notes."

Before the bullies arrive, Ah-young finds her notes (which was actually the ripped paper used in the ghost game) and escapes through the window, Jung-woo jumping out first.

Though she considers the song to lack emotion, Ah-young is secretly touched and offers Jung-woo her help in the production of his piece.

Later, Jung-woo learns that Yeok-ho is orphaned for long and that he trained under Ah-young's older brother, the late Yang Young-woong (Lee Jong-hyun) who died after saving people during a severe flood in Namil years ago.

After the boxing match, Jung-woo braces himself for the upcoming National Singing Contest, with the help of Deok-won and his fellow schoolmates as the musicians.

Yeok-ho, who was attending another boxing training, arrives late at the venue and finds out the Ah-young had come to watch the contest ahead of him, though both of them were not able to see the performance.

In Seoul, Ah-young gets teary-eyed in happiness upon watching Jung-woo's song revealed in a TV replay of the National Singing Contest.

The results of the licensure examinations are released and Jung-woo is one of the lucky passers, prompting him to continue his career in Namil High School.