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Jin-Ho Myung returns a wallet and cell phone to a beautiful young woman (Hae-Young Nah) in a book shop, little realizing that she would find out that he has already (rather presumptuously) taken 7000 won to buy a book he really wanted. The woman in question is understandably rather upset but decides to consider the money as a loan, which she expects to be paid back.
On the first day of the new school year, he learns, to his horror, that the very same woman is to be his new homeroom teacher. The bad news for Jin-Ho is that she remembers their previous encounter very well.
As the school year progresses, Jin-Ho and Hae-Young's relationship grows and develops despite Ji-Soo (Jin-Ho's classmate and their class president) and Jae-Kyung (Jin-Ho's old friend and former president of the Film Studies Club) having feelings toward Jin-Ho. Caroline, Hae-Young's half-sister from England, also makes her move towards Jin-Ho but she understands Jin-Ho's love to Hae-Young.
Currently, Jin-Ho denies Jae-Kyung's confession of love. Jae-Kyung regretfully accepts Jin-Ho's decision on loving Hae-Young instead of her. As Jin-Ho leaves the Film Studies Club room, Jae-Kyung cries in total depression. She soon has a mental breakdown, in which she wears her high school uniform and goes back to school, reminiscing her old days as the president of the Film Studies Club and thinking about Jin-Ho. Jin-Ho soon learns about Jae-Kyung's condition and she is rushed to the hospital. In hopes of bringing her back to normal, the doctor suggests Jin-Ho to support and love her in order for her to recover much faster. Later, Hae-Young arrives at Jin-Ho's house, telling she will do anything in Jin-Ho's desire. Jin-Ho, knowing Jae-Kyung's mental condition, becomes hesitant and suddenly did nothing which makes their relationship split apart. As Hae-Young breaks down in tears, Jin-Ho goes to the hospital, telling Jae-Kyung (who is now recovering) that he and Hae-Young had finally broke up. Jae-Kyung hears Jin-Ho's tearful confession, and relies on her hope that Jin-Ho might choose her.
Following her break-up with Jin-Ho, Hae-Young decides to resign from her post after the first semester of school. Ji-Soo later asks if Jin-Ho was aware of this fact and if Jae-Kyung was involved, but Jin-Ho denies the involvement of Jae-Kyung and claims that he no longer loves Hae-Young. Ji-Soo later goes to see Jae-Kyung where she overhears her phone call, saying that she had already recovered. After hearing the phone call, Ji-Soo confronts Jae-Kyung, who claims that she doesn't know Ji-Soo, who beats her out of anger for tricking Jin-Ho.
Hae-Young eventually goes to the airport to leave Korea, which she confesses to Caroline that she's happy to be her sister. When she arrives in the UK, she is confronted by Jin-Ho who hands her a device resembling an iPad containing a video message from Jae-Kyung.