A working class girl winds up at an exclusive prep school. Unassuming high school girl Jan Di stands up to — and eventually falls for — a spoiled rich kid who belongs to the school's most powerful clique.
Jan-di is an average high school girl whose parents operate a Laundromat. One day she delivers laundry to Shin-hwa high school, where the top 1% of the wealthy children gets to go, and accidentally saves a student from a suicidal attempt. After this incident, she is scouted to this prestigious school as a swimming athlete.
Jandi gets Jihoo’s help when students attack her at the locker room. Jandi, cannot stand Junpyo’s smugness, goes to see Junpyo and kicks him right in the face. Rather than paying her back, Junpyo finds his curiosity from this girl who reminds him of his older sister, Junhee. After school finishes, unknown guards kidnap Jandi. She wakes up and notices the unbelievable things that are happening right in front of her eyes…
After came back from the school trip, Junpyo’s curious feeling for Jandi grows even bigger. Jandi meets Jihu, who is having a hard time because of Seohyeon, and Junpyo hears about this. Jandi worries about Jihu and goes to see him, but encounters Junpyo who is waiting for her with jealous eyes. At Seohyeon’s birthday party, Seohyeon announces that she’s going to France, leaving everything behind. When Jandi goes to say goodbye to her, she sees the scene of Jihu declaring his love for Seohyeon.
While Jandi and Junpyo are out on a date, they accidently lock in the cable car. Junpyo suffers from high fever and Jandi nurses him all night. This quickly spreads to the whole school and Jandi experiences totally different treat from fellow students, as Junpyo’s girlfriend. Jandi’s friend Minjee brings her to the club and she finds a guy who reminds her of Jihu. The next day morning, Jandi wakes up in the hotel room and she can’t remember what happened the night before.
Junpyo believes in Jandi’s innocence and treats her wounds. This moves Jandi’s heart, and Junpyo finally announces that Jandi is his girlfriend. At that moment, Jihoo, came back from Paris, appears in front of them. During the weekend, Junpyo and the F4 take Jandi and her friend Ga-eul to New Caledonia. In there, Junpyo takes Jandi to breathtaking sceneries, but Jandi’s mind is still on Jihoo.
Thinking of the reminiscene with Seohyeon, Jihoo gets depressed. While Jandi is comforting him, Jihoo kisses Jandi. Junpyo bursts with rage and jealousy, and returns to Seoul alone. He announces that he is going to dismiss Jihoo from F4 and expel Jandi from school. However, the unexpected supporter Junhee, the sister of Junpyo, appears in front of the two.
Junpyo and Jihu square off in a sports match. The fate of Jandi and Jihu hangs on this game. They compete in horse-riding and racing, and Junpyo grows more jealous as he watches Jandi roots for Jihu. Before the final battle begins, Junpyo goes to meet Jandi.
At last Jihu and Jandi go on an official date. Invited to Jihu's house, Jandi smiles bitterly at the remnants of Seohyun found all over the house. Tailing the two, Junpyo discovers that he still has feelings for Jandi. Meanwhile, Jandi rushes to the hospital upon hearing that Junpyo has been in an accident. There, she realizes her true feelings for Joon-pyo.
Junpyo and Jandi keep fighting and making up, and they get to know each other. However, Junpyo’s excessive presents makes Jandi hard to keep up with his pace. She continues to avoid Junpyo and Jihoo comes to the swimming pool and cheers her up. Jandi and Junpyo go out on a date with Gaeul and her new boyfriend. However, Jandi and Gaeul’s boyfriend get uncomfortable to each other.
Yijeong unexpectly sees Gaeul on the street and helps to revenge her bad boyfriend. Jandi clears up her misunderstanding about Junpyo, and the two go out on a date on Valentine’s Day. They bump into Junpyo’s mother, and Junpyo tells her not to do anything bad to Jandi. The mother, who doesn’t even think of F4 as the friends of Junpyo, starts her next action to cut Jandi off from Junpyo as she thinks Jandi is ridiculously improper to her son.
Jandi starts a part time job as a model on the cover of textbook, only to find out that she’s been deceived. In the dangerous moment, a mysterious man comes to the rescue of Jandi, and soon vanishes. The man turns out to be Jeha, a freshman at Shinhwa High School. Jandi and her purported fan, Jeha become friends. Unhappy with Jandi, who spends all her time working at her part-time job, Junpyo finally explodes at seeing Jandi on the magazine cover with model Haje. And Jandi finds the second “Red Card” in her locker.
Jeha kidnaps Jandi and Junpyo goes to rescue her alone. Jeha and his friends beat Junpyo, but Junpyo bears it. F4 members get into the scene, finish off the situation. When Jeha tries to hit Junpyo again, Jandi protects him. The two are hospitalized, but find out the feeling for each other again. The friends go on a ski trip together, and Junpyo gives Jandi a custom-made necklace. However, Jandi accidently loses it and braves the snowstorm to look for it.
Time has passed by; F4 are now college students and Jandi is in her senior year at high school (12th grade). Jandi keeps practicing swimming and works part-time, and awaits Junpyo. Jihoo suggests her to record her message in a video and send it to Junpyo, and he also takes her to the hospital and checks the condition of her shoulder. Jandi finds out that she won’t be able to swim anymore and now it’s too late to cure. Confused and toubled, Jandi goes to Macau to meet Junpyo. She gets surprised at the magnificent look of Hotel Shinhwa, but doesn’t get overwhelmed. However, when Jandi finally gets a chance to see Junpyo, he only gives her the cold shoulder.
The F4 comes to Macau and joins Jandi. They meet Junpyo and ask why he’s ignoring Jandi, but he only says that Jandi is no longer related to him. Jandi acts as if she’s alright, but can’t stop crying in front of Jihoo who looks like understanding everything. Jihoo helps the two to meet up, but Junpyo reconfirms the end of their relationship. When Jihoo and Jandi are about to go back to Korea, they unexpectedly meet Jihoo’s friend and were invited to his house in suburbs..
Back from Macau, Jandi is having a hard time trying to brace herself up. The master of the restaurant where Jandi works, asks her to visit the old doctor’s office, who often comes to the restaurant, and this gives Jandi a fresh impression. Jandi talks to Junhee about her true feeling and Junhee asks her mother not to make Junpyo a victim. Junpyo comes back to Korea and goes to Shinhwa University. The mother invites Jandi to Junpyo’s birthday party. In there, she hears about Junpyo’s engagement with Jaekyeong, the girl who Jandi met in Macau.
After the birthday party, Jaekyung shows her interest to Junpyo. With her easy going personality, Jaekyung becomes friend with Jandi. Although Jandi likes Jaekyung’s untainted and friendly attitude, she feels uncomfortable. Jihu runs into his grandfather while taking saddened Jandi to her volunteer work at the clinic. Yijung and Woobin fail to seclude Jaekyung from Junpyo. With Gaeul’s help, Yijung finally reunites Junpyo and Jandi.
As the merger of Shinhwa group and JK group begins, Junpyo and Jaekyung go out on their first date. However, Junpyo reminds of Jandi every place Jaekyung wants to go. Jaekyung senses it and asks Junpyo to kiss her as the lover. Jihoo and Jandi see this while they are talking on the stairway… Yijung witnesses his father with a woman at his workroom and has a big argument. Suffering deeply, Yijung thinks of Eunjae, his first love. Gaeul signs up for the pottery class and meets Cha Eunjae, the student teacher, in there.
Come back from the resort, Jandi and Jihoo visit Buddhist temple to clean their mind away. However, when Jandi returns, she finds out that her parents have decided to move to a fishing village to earn money. Junpyo wakes up and finds out that Jaekyung has prepared him the breakfast. Jaekyung says that she always dreamt of this, and Junpyo feels pity for her. Yijung tells junpyo that Jandi stopped swimming because of her shoulder injury, and Junpyo feel guilt and regret. Jandi moves to a rooftop house and is surprised when Junpyo unexpectedly shows up as her neighbour. Gaeul decides not to hide her feeling anymore and asks Yijung out on a date. Yijung rejects it but comes back to see her few days later.
Jihu runs out of the clinic and wanders in the rain. Jandi's heart aches for Jihu, as she nurses him all night long. At the same time, Junpyo awaits Jandi at the rooftop house. Yijung takes Gaeul out to the club, chats with other ladies, and makes her feeling uncomfortable. However, already knows the pain in his heart, Gaeul pities him instead. After finding out the whereabouts of Junpyo, his mother confines him to the house. Then she goes to Jandi to humiliate her in front of everyone. The next day, F3 and Gaeul visit Jandi to comfort her and they start rooftop room renovation. Jaekyeong takes Junpyo out from home and joins them, and when they all start ‘the truth game’, the tension starts rising.
Jandi becomes Junpyo’s personal maid and controls over Junpyo’s naughty behavior. While she was in Junpyo’s room to wake him up, Jaekyung and Junpyo’s mother enter the room and get shocked to see her. The mother wants to kick her out from the house immediately, but the head maid and Jaekyung intervene in. Seokyeong collapses at the clinic and Jandi takes him to Jihoo’s house. Jandi tries to reconcile the two and now they start to live under the same roof. Gaeul now knows that Eunjae was Yijung’s first love, and tries her best to bring them together.
Junpyo’s mother rushes the wedding, and Jaekyung asks Jandi to be her bridesmaid. On the day before the wedding, F4, Jandi and Gaeul arrived at Jeju Island for the wedding, and the complication between Junpyo, Jaekyung, Jandi and Jihoo reaches the climax. Junhee advises Junpyo not to do something that he is going to regret. Junpyo goes down on his knees and ask Jaekyung to break off the engagement.
The wedding ceremony finally starts and Junpyo enters in. However, Jaekyung maps out a plan and ruins the wedding. Junpyo and Jandi sneak off to the resort and spend time together. Jaekyung returns the necklace to Jihoo and leaves to New York on the next day. Jandi and Junpyo’s happy moment is about to fall apart, since the break off brings a crisis to Shinhwa group. Now Junpyo’s mother anger is towards Jandi.
With Gaeul’s effort, Yijung now finds out Eunjae’s declaration that he had missed three years ago. Gaeul brings Yijung to the rooftop, uncovers his eyes, and the message magically appears in front of him. Junpyo’s mother finds out about Junpyo and Jandi are secretly dating, and starts to attack the people around Jandi. Finding out who fall them into a difficult situation, Jandi makes up her mind.
Jihu finds out Jandi’s whereabouts. He expects Junpyo to come with him, but Junpyo disappoints Jihoo again by refusing to go. Jandi arrives at the fishing village, but people expect her as the fiancé of the heir of Shinhwa Group. When it reveals as false, Jihu suddenly appears in front of her and settles the situation. Jihu declares his feeling for Jandi that he suppressed for long time and Junpyo looks at them from far back.. and suddenly!
Jandi leaves a lunch box for Junpyo, who lost the memories about her. Jun-pyo almost remembers something about Jan-di as he eats the lunch, but Yumi lies takes crushes Jandi’s last hope. Jandi and F4 are invited Junpyo and Yumi’s pool party. In there, they hear surprising news.