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Jeong Yoojin is your regular "Little Miss Tardy" who too often finds herself in trouble for her lateness. Today seems no different and once again it is another rush to school for Yoojin with her childhood friend; Kim Sanghyeok. Unbeknownst to her, this day is different. Fate intervenes as she makes the acquaintance of a mysterious new student.

Kang Joonsang and Yoojin's love begins to blossom as they keep on running into each other. In the meantime, Joonsang becomes confused and shocked to find the same picture he has at Yoojin's house.

Yoojin waits for Joonsang until late at night, but he never appears. The next day, she hears the shocking news that Joonsang was killed in a car accident. Ten years later, Yoojin is engaged to Sanghyeok. She meets her friend Oh Chaerin's boyfriend Lee Minhyeong, who looks just like Joonsang.

Yoojin and Minhyeong work together in renovating a ski resort. Although Yoojin tries to hide that she's working with Minhyeong, Sanghyeok soon finds out.

Sanghyeok is unhappy that Yoojin is working with Minhyeong but realizes that he cannot make her quit the work. When Chaerin finds out that Yoojin and her boyfriend work together, she feels threatened and makes up lies about her to Minhyeong.

Minhyeong takes the drunk Yoojin to his hotel room and is surprised when she calls him "Joonsang". The misunderstanding between the two deepens and Chaerin sets fire to Minhyeong's anger towards Yoojin.

Yoojin is injured while trying to save Minhyeong, and her boyfriend Sanghyeok is infuriated. When Minhyeong realizes that his girlfriend Chaerin lied to him about Yoojin, he asks her for some space in their relationship. Minhyeong and Yoojin go together to a pond where she shared beautiful memories with Joonsang when they were in high school.

Minhyeong and Yoojin spend the night at the ski resort and Sanghyeok and Chaerin wait for them anxiously. The next morning, Sanghyeok questions Yoojin whether she truly loves him. When she is unable to answer him, Sanghyeok coldly turns away from her.

Yoojin tells Sanghyeok that she cannot marry him, and at the same time, she assures Minhyeong that she cannot have a relationship with him. She begs Sanghyeok to give her a chance to stand on her own. Sanghyeok does everything he can to change her mind, while stepping up to Minhyeong and warning him to not shake her heart.

Yoojin finally breaks up with Sanghyeok when she realizes that she is completely in love with Minhyeong. Although everybody around her opposes her love, she tells herself that she can be happy as long as she is with Minhyeong. Meanwhile, Sanghyeok refuses to eat or sleep.

Yoojin tells Sanghyeok about Minhyeong’s mother. Sanghyeok goes to the high school to see student records.

Minhyeong goes to Joonsang's old house and meets Kang Mihee. Mihee assures him that Minhyeong is actually Joonsang. Sanghyeok calls Minhyeong as Joonsang and warns him to stay away from Yoojin. Meanwhile, Minhyeong takes Yoojin to his car by force, and tells her that he is Joonsang. Unable to believe his words, Yoojin only looks into his eyes with sad tears.

Despite Minhyeong's confession that he is Joonsang, Yoojin does not believe him, thinking that he is only doing that to get her back. Meanwhile, seeing Yoojin even in his missing pieces of memories, Minhyeong sadly sheds tears. Minhyeong decides to leave as he realizes that as long as he cannot fully remember his past, he can no longer linger around Yoojin.

When Yoojin finds out that Minhyeong is Joonsang, she desperately runs out to find him and stop him from leaving. During her attempt, she nearly gets into a car accident, and Minhyeong jumps in and gets hurt instead.

Joonsang recovers his lost memories partially and shares them with Yoojin. Although Yoojin and Joonsang spend a happy time together for the first time in ten years, they both have heavy hearts as they are unable to erase the pain, they have caused their friends.

Joonsang and Yoojin finally make up with their friend. One day, Joonsang realizes that the last missing piece of his memories concerns his father. Meanwhile, when Professor Kim Jinwoo hears that Minhyeong is Joonsang, he cannot help but question whether Joonsang is his long-lost son.

Yoojin and Joonsang decide to get married and head for Chuncheon to her mother's house to get her approval. However, when Yoojin's mother finds out that Joonsang is Mihee's son, she firmly disapproves of them getting married.

Unable to change Yoojin's mother's mind, Yoojin and Joonsang decide to have a secret wedding by themselves. However, their plan is shattered when Sanghyeok chases after them.

Yoojin discovers a shocking secret from Mihee. Meanwhile, Joonsang treats Yoojin coldly, knowing that he should cut ties with her. Realizing how difficult it must be for him, Yoojin suggests to Joonsang that they go their separate ways.

Professor Kim finds out that Joonsang is his lost son. Joonsang hears shocking news from the doctor that unless he undergoes brain surgery, he will have a very slim chance of living due to the aftermath of his car accident.