The film centers around Bo-ra (Kim You-jung), a 17-year-old high school student who is athletic and has a bright personality. One day, Bo-ra finds out that her best friend Yeon-doo (Roh Yoon-seo) has a big crush on Hyun-jin (Park Jung-woo). But before Yeon-doo gets a chance to become friends with Hyun-jin, she has to go to the US for heart surgery. Bo-ra promises Yeon-doo that she will keep an eye on Hyun-jin and find out everything about him while she is away.

To secretly spy on Hyun-jin, Bo-ra becomes close with Woon-ho (Byeon Woo-seok), Hyun-jin’s best friend, and develops feelings for him.

During a press conference held at CGV Yongsan on Wednesday, the director said this movie’s story all started with the discovery of a shared diary that she used to write with her friend when they were students.

Called an "exchange diary," friends take turns writing their thoughts or comments in a shared notebook. Such diaries were popular among students in Korea back in the 1990s.

“My friend who is now a mother said she accidentally bumped into her first love. We talked about that incident in our chatroom and it reminded us of the diary that we used to write together,” the director said.

In the old diary, the director found out how she used to write about her friend’s first love after observing him. And that was the inspiration for the new movie.

The director explained that she set the film in 1999 because it was a time of change.

“It was a time when some of us started using the internet. Some had pagers, some had cellphones and some had neither. It was a time of rapid change and I think it also is similar to the situation of Bo-ra who is going through puberty,” the director said.

The actors talked about some of the memorable props that were used in the movie.

“I saw floppy disks for the first time when filming. I thought they were CD cases that were piled up at first and thought it was really interesting,” actor Kim said.

“In Hyun-jin’s room, there was an old MacBook computer that was used at the time. I really wanted to take it home,” Park said.

Byun said the video store that Bo-ra’s family owned reminded him of the time he used to watch videos with his family every weekend.

The film also introduces many pager codes -- a system for sending messages using only numbers -- that young Koreans used in the 1990s.

“I was so shocked that the cast did not know about pager codes,” director Bang said.

The director said even some of the staffers did not know the pager code "1010235," which often appears in the movie. It means "I passionately love you."

“The staff thought that it was Hyun-jin’s pager number. That was when I realized that it is now a secret code that only people of my generation know and I considered explaining it with subtitles in the film,” she said.

During the press conference, the director and actor Kim also talked about the film being screened at the 27th Busan International Film Festival earlier this month.

“It was the first time we watched the film in a theater so it was great. I got positive energy from the audience who told us that they really enjoyed the film,” Kim said.

“It was an honor for me -- and also the actors -- to premiere the film at the fest,” the director added. “It was a great start and I hope that this energy can also lead to the success of the film.”

The movie will be on Netflix on Friday.