In 1997, nothing was more important to Shi Won than the Korean boy band 'H.O.T.' She and her five closest friends exemplified the teenage angst of K-pop idolatry that was escalating during the decade. They reconnect 15 years later and relive their youth of the late 1990s.

Yoon Jae has circumcision. It is supposed to be a secret, but the whole town knows, including Shi Won. Shi Won accidentally hands him a pad as she helps him search for his things. This scene shows how they are physically developing into adults. But the changes are gradual, and they still fight like children. Shi Won lies at school about her homework, and Yoon Jae does not help her out with the lie. So she is punished, and she is angry with him. They have a fight, and accidentally Yoon Jae touches her too intimately. The incident makes Yoon Jae see Shi Won as a woman. Yoon Jae unfortunately realizes that although he has become a man, Shi Won has not become a woman yet.Yoo Jung secretly falls in love with a member of an idol group that is the rival of H.O.T. called Sechs Kies. Shi Won considers this betrayal, and it angers her. This causes a rift in their friendship.

Shi Won goes to Seoul and camps in front of Tony An's house. She finally returns home after getting her T-shirt signed. When she returns home, her father cuts her hair as punishment. Since then, her parents fight regularly. She confides in Joon Hee about her worry about their quarrels. Yoo Jung also cuts her hair to show moral support for her friend. This act restores their friendship. A new student from Seoul transfers to Gwangan High School. He may be Yoon Jae's rival as the best looking guy in school. He is good-looking and cool. He is macho, and he impresses the other boys. But in front of girls, he is extremely shy. Somebody under the name 1004 (angel) sends Yoon Jae a music file. He thinks it is from Shi Won, but she is not interested in him at all. In fact, she talks badly about him to Joon Hee.Shi Won and Joon Hee chat online and exchange big secrets. Shi Won realizes that what she knows to be true is actually not true at all.

It is the World Cup game: Korea vs. Japan. Shi Won's friends come over to watch the game together, but she forgets to order chicken. Luckily, the delivery man gets his orders wrong, and Shi Won and the group decide to eat the chicken that was ordered for somebody else. When Korea scores a goal, Yoo Jung hugs Hak Chan, and Shi Won hugs Joon Hee. Yoon Jae is suspicious of the closeness between Shi Won and Joon Hee. He is done with fair play. He realizes that one must be proactive to get what one desires. Shi Won gives her phone number to the Seoul boy she has been chatting online with. He says he will call after the game. Tae Woong falls in love with Song Joo, whom he tutored. They decide to marry. One day he goes to watch the Busan Seagulls game and sends Song Joo on a retreat. But there is an accident. We find out that she was Shi Won's sister, and Shi Won now behaves just like her towards Tae Woong.

Life's counterattacks include Yoon Jae's broken arm, falling in love with Shi Won, a street vender's rags-to-riches story, her father's cancer, and the IMF crisis. The only way to go about life is fighting it and just going on with life. Yoon Jae breaks his arm and is hospitalized. Shi Won sleeps by his side with her glasses on. Yoon Jae notes that he has fallen in love with her over trivial things such as her glasses.A street vender finally finds his parents. He goes from rags to riches. This case shows that there are many ups and downs in life.Shi Won asks her father to buy her expensive jeans, but he refuses. So she writes sad stories and sends them to a radio show in hope to win brand name jeans. Yoon Jae warns her that words come true, and her story about her father having cancer does come true.Shi Won has huge homework that she cannot complete in time. Her father completes it for her, and Shi Won is touched by his love and care.

Shi Won's father has gastric cancer and is in the hospital. Everybody in his ward watches a popular drama until the main character finds out he is dying from cancer. This depresses all the cancer patients. Shi Won's mother starts calling the drama writer to make her change the story line.Shi Won writes Tony An a fan letter in her own blood in order to become a board member of H.O.T.'s fan club. But when she hears that Tony An is disgusted by it, she is crushed. Yoo Jung devises a way to go see a basketball game with Yoon Jae alone. But Yoon Jae asks for the tickets so that he can cheer up Shi Won. Yoo Jung is crushed, so Hak Chan tries to cheer her up. She ends up kissing him. At the basketball game Yoon Jae tells Shi Won that it bothers him to see her so close with Joon Hee, but she does not hear him. The grown up Yoo Jung and and Hak Chan announce their wedding plans at the reunion. Shi Won is pregnant, so she cannot drink.

Yoo Jung repeatedly breaks up with Hak Chan. Each time Hak Chan has changed his habits to be more accommodating. This time he does not know the reason for the break up. Yoon Jae and Joon Hee, the top two students, plan to enter the Air Force Academy. Yoon Jae wants to become a pilot because as a child he hears that Shi Won wants to marry a man in uniform. Joon Hee wants to go where Yoon Jae goes. But Yoon Jae does not make it. He later becomes a judge. Joon Hee appears to also not have made the physical exam for the Air Force Academy.Shi-Won's fan fiction stories reveal her literary skills. She wins a writing contest and gets into Dongguk University. She is happy to enter the same university where Tony An is enrolled at. Eventually she becomes a screen writer. Tae Woong as a teacher encourages students to pursue dreams. He also wants to pursue his real dreams, so he gets ready to resign. His mother had wanted him to become president of Korea.

It is finally the D-Day for the college entrance exams. Afterwards the students make elaborate plans. Seong Jae wants to pick up girls, so he goes to a karaoke bar with Joon Hee. They try to pick up girls, but one girl turns out to be his sister. Hak Chan invites Yoo Jung to his house to watch a videotape he has made for her. She thinks he wants to seduce him into bed, so she honestly tells him she has her period. Yoon Jae wants to meet Shi Won after the exams. So does Tae Woong. Yoon Jae carefully plans to tell Shi Won that he likes her and buys her a dog. Tae Woong wants to take Shi Won out for a drink and also tell her that he likes her. Yoon Jae is shocked to find out his brother's plans. He realizes that the most important thing about D - Day operations is scouting the field, which he did not do thoroughly.

Yoon Jae finally reveals to his classmates that their math teacher was his brother. They are shocked and had no clue because the teacher had been using a false last name. Shi Won's father warns Yoon Jae to meet a sweet woman or he will end up just like him. Shi Won's mother ended up with her husband because of a misunderstanding. But couples end up as fate would have them. Yoon Jae gives the dog to Joon Hee, who gives the dog to a child in the park. But the dog ends up in Shi Won's house as originally planned. Tae Woong starts up an online site where people can find their old classmates and becomes very successful. He asks Shi Won to date him and buys her a phone. Yoon Jae tells Shi Won to stop behaving childishly when she hugs him. He is trying distancing himself from her.

"The reason why I like you is because you're you. Could there be another reason? I wish I knew. Then I could figure out how to stop liking you." People are asked this question and there is no better answer. Sung Jae falls in love with Gak Ha after seeing her dance but tells her too honestly about his reason for liking her. Shi Won thinks Tae Woong likes her because she looks like her older sister. Joon Hee and Yoon Jae ask each other why the other likes Shi Won. Joon Hee confesses that he likes Yoon Jae, but Yoon Jae thinks he is joking in order to avoid the question why he really likes Shi Won. The Golden Disc Music Award is held in Seoul, and there is great tension between the H.O.T. fans and Sechs Kies fans over who will win the grand prize.

Hak Chan keeps avoiding meeting Yoo Jung's friends. Finally he shows up and pays for the bill, which makes Yoo Jung's heart melt. But when Hak Chan does not introduce her to his mother as his girlfriend, Yoo Jung breaks up with him. Shi Won's parents go on a cruise vacation leaving Shi Won alone. On her way back home from working at the convenience store, a man follows her. Afraid, she repeatedly calls Yoon Jae, and Yoon Jae repeatedly calls her. But they do not get connected until she calls his home phone. Yoon Jae dashes to where she is, bleeding with only one slipper. Shi Won realizes that their relationship is not simple friendship, but it is a complicated relationship between a man and a woman.

Shi Won's parents are worried that she will have adjusting difficulties in Seoul, but Shi Won is excited to go to Seoul and has been packing in advance.Yoo Jung is suspicious of Hak Chan and Dan Ji. Hak Chan has been distant. He finally tells her that he is going to Hawaii to study. Yoo Jung tells him that all she wants is that he communicates with her.It is Shi Won's birthday, so there is one last party before everybody parts ways. Shi Won told Yoon Jae the year before that he should tell a guy to buy her a ring for her 18th birthday. This year, she asks Yoon Jae if he has a ring for her. He tells her his feelings and that she is cruel to ask for a ring from him. She asks to be friends, but he tells her he never wants to see her again. Tae Woong gives Shi Won a ring, but she tells him that she needs to talk.

Yoo Jung's father passes away. She realizes that there are no next times. She should have been good to him when he was around. Her friends come to the funeral. Even Hak Chan finally shows up. Shi Won runs into Yoon Jae in a coffee shop. He had avoided seeing her for 6 years. She asks if he has a girlfriend and catches him lying. Shi Won tells him that she likes him. Yoon Jae hesitates to answer, even though he knows that there is never a next time. Meanwhile, Tae Woong has been Daddy Long Legs to Shi Won throughout all this time. He asks her if Shi Won knows how the ending of the story goes, telling her that he wants more. Shi Won does not want more from him. Even 6 years ago, Shi Won told him that she likes somebody else. Yoon Jae overhears Shi Won and Joon Hee talk about both of their crushes on him. He is beyond shocked.

It's what the heart demands. So do what the heart tells you to do. Joon Hee tells Yoon Tae that he should not make his brother the excuse for not being honest about his feelings. People's hearts do not choose who they like. Also he tells him that he should not be bitter towards her for not knowing that Yoon Jae liked him all this time, because some people may not know ever. Joon Hee lies to Yoon Jae about why he needs to move out. Tae Woong is hospitalized to remove his polyp. He meets his surgeon in funny way. She farts and takes her stockings off not knowing he is in the room. He finds out that she will remove his polyp in a most embarrassing way.Shi Won says that if Yoon Jae does not tell her if he likes her, she will kiss him. Yoon Jae kisses her first. Later, Shi Won and Joon Hee talk on the phone afterwards about it. Tae Woong overhears the conversation and is shocked.

Tae Woong asks Yoon Jae to meet a girl. Yoon Jae suspects that it is a blind set his brother is setting him up on. He tells him not to set him up like this again. Tae Woong tells Yoon Jae that he knows about them. Yoon Jae tells him that he gave up Shi Won back in high school for the person he loves the most, his brother. But Tae Woong tells him Yoon Jae that he cannot give up on Shi Won even for his brother. After much anguish, Yoon Jae tells his brother that he will not give up her for his older brother again. Shi Won tells Tae Woong that she has something to say to him. She feels she cannot go on ignoring his advances because he is a dear person to her. Back into the future, Shi Won is having a baby. Both Yoon Jae and Tae Woong come running.