One blazing summer day, Ja-gyung, a sixteen-year-old girl, is pissed off with her dad's affair. She barges into her dad's mistress' house and encounters her two little kids.

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Han Oh-ki is a weakling. One day, he is working out to become a real man, when multi-sportswoman Chae-in saves him from cramps at the swimming pool. He falls in love with her at first sight and tells her how he feels when they get close. However, he's nothing but a helpless younger brother to her and gets rejected. Oh-ki is mad and challenges her to a sports race and if he wins, she has to acknowledge him as a man...