From the time when they met at the age of five, and even now in high school, Kwon Yuri and Jung Yunho can't stand each other. Yuri is hot-tempered, violent, and feared by all the students at school, while Yunho is two-faced, conceited, and loved by his "fans" who don't know his real personality. The two, who can't spend five minutes with each other without arguing, try to stay as far away from each other as possible. It isn't too difficult, considering Yuri is part of the swim team and Yunho leans more to the basketball side. They've been fine, ignoring each other, though they do make nasty comments about each other behind their backs.

Until their siblings started dating.

Yuri is shocked when she finds out that her sweet, innocent, and pure younger sister, Seohyun, is dating the likes of Yunho's younger brother, Changmin. It's unthinkable to her. What exactly could've made her sister fall for that devil's younger brother? 

Suddenly she and Yunho are forced to spend time together. From their siblings planning a double date, to spying on their siblings secretly, and even trying to break the happy couple up. Kwon Yuri realizes Jung Yunho is not such a bastard. Jung Yunho realizes Kwon Yuri really is not a flat, ugly, monster. And both of them are so very confused about their feelings. 

They hated each other for 12 years of their life. It's impossible to like each other in that way now, right?