Kang Ho-gu has never had a proper girlfriend, with girls constantly taking advantage of his sweet, naive nature by enjoying all the perks of dating then friend-zone-ing him. His relationship-savvy twin sister Kang Ho-kyung mocks him and his best friends Kim Tae-hee and Shin Chung-jae for being sorely lacking in any dating skills.
Do Do-hee is an athlete on the Korean national swimming squad, and won the silver medal at the recent Asian Games. Driven and competitive, she's frustrated at never getting first place and is known for cursing a lot.
The timid, bullied Ho-gu had a crush on Do-hee in their teens, when her beauty and sports cred made her the most popular girl in high school. She's the only reason he goes to their high school reunion, and when they meet again, he's surprised to learn that she remembers him. Ho-gu grabs the chance to spend time with the girl of his dreams, not knowing that Do-hee has a secret, that she's pregnant possibly by her ex-boyfriend Byun Kang-chul, their class president and now a successful lawyer.