According to Myungsoo, promises aren't meant to be broken, but meant to be fulfilled or kept. In his case, he never broke a promise ever. Because of that, he became obsessed with things. And by things, it's about protecting his little sister, Minsoo. Like watching her from a distant whenever she is in the classroom, eating with her when it is lunch time, walk her home, follow her everywhere and do the craziest things like watch her sleep. Try to hurt her and Myungsoo will make you stay in your house forever. Not even kidding.

Because of her brother's scary attitude, she barely have friends. Scratch that, she really don't have friends. Minsoo was very pretty that boys drool over and is very nice. But no one dared to ask her out or talk to her. Once, a new student decided to ask her out and the next thing the school knew, he was in a hospital with broken bones. That's how crazy Myungsoo is. Most people are worried about Minsoo. They want to take her away from Myungsoo but they can't since he will hunt them down, really. So beware.

Years will pass and all the people around the world will grow up. They might also fall in love with someone or something along the journey in their life. In Minsoo's case, she is one of them. She fell in love with the boy who is secretly courting her whenever they are in class. Without her brother's knowledge, she accepted him. What will happen if Myungsoo found out their secret relationship? This could go crazy. But the question is, what will the crazy obsessed brother do to the boy who is dating his precious little sister?