Shin Sol-i isn't shy about her unrequited crush on Cha Heon, her next-door neighbor. One day, she gets an idea to write him the perfect love letter.

Woo Dae-sung ditches swim practice before a competition for a fun night out at karaoke. When Heon is down, Sol-i wants to be by his side.

Sol-i declares she's running for class representative, surprising her friends. While Dae-sung shows his support, Heon disappoints her.

Desperate for prize money so she can buy Heon a birthday present, Sol-i pours all her energy into winning at the school's sports competition.

The class prepares to audition for the school play. Sol-i wants to act out a romantic story with Heon, but he's not easy to convince.

After the incident at the play, Sol-i begins to avoid Heon and considers a big decision. To cheer her up, Dae-sung puts together something special.

Excited to try out for a TV singing competition, Jeong Jin-hwan asks the friends to come and support him. Sol-i worries when her parents fight.

Heon agrees to spend a Saturday with Sol-i, who wants to take him to see his favorite soccer team. But his mood soon sours.

With the flu going around, Sol-i frets when she has a cough and is sent to the school nurse's office. Heon looks out for her in his own way.

It's almost Christmas! While Sol-i racks her brains on what to get Heon, her parents wonder whether their daughter has a crush on someone.

When the class goes on a trip outdoors, Sol-i plans a surprise event for Heon. Jin-hwan ruins Kang Ha-young's night at the movies.

Knowing he's hurt Sol-i's feelings, Heon attempts to apologize. A visibly troubled Oh Hui-ji acts out of character.

Hui-ji steals from the school nurse's office. Jin-hwan stops coming to school when he finds out about Ha-young's crush on someone else.

Sol-i learns that Heon wants to attend a prestigious university. Dae-sung's worsening arm injury threatens his swimming career.

Heon's plans for his future don't align with what his mother wants. When Dae-sung's grandmother goes missing, Heon and Sol-i also look for her.

As all her friends get accepted into college, Sol-i feels down about herself. Heon encourages her to keep trying, and promises he'll help.

Sol-i is elated when Heon makes their relationship official. Not wanting to worry Ha-young, Jin-hwan keeps a secret from her.

Struggling in her job search and with putting up with Heon's demanding work schedule, Sol-i snaps when she finds out he may be leaving for America.

Hearing her dad's been hospitalized, Sol-i hurries over — and bumps into Heon, who's back in Korea. All the friends gather for drinks.

Sol-i calls Heon about his actions the night before. Everyone comes together for Ha-young and Jin-hwan's wedding.

Heon shows up to a gathering in his old school uniform, and gets honest about his feelings. Dae-sung decides it's time to bare his heart to Sol-i.

A lovely surprise awaits Sol-i outside of Heon's hospital. The next day, Sol-i's parents come to visit her without notice.

Looking back on all they've endured together, Heon recounts for Sol-i how he came to fall in love with her.