You know how it is with kids. When a boy likes a girl he teases the girl to get her attention and makes her life a living hell. It's viewed as natural. 

However, Kai, aka Kim Jongin, took that principle to a whole new level. He made it his life's mission to make Park Jaein, his neighbor, cry everyday. He lived to see her cry. He did everything normal boys did to get a girl's attention. Worms in her lunchbox. Stole her cookies. Sprayed water on her when she woke up at the end of nap time. Slid ice down her back. Pushed her into a pool with all her clothes. Check, check, check, check, and check. 

When an unexpected event causes Kai to fall into misery, Jaein is freed from Kai's torture, and finally opens her eyes to the ugliness and pain of the world. It makes her cold.

While Kai was in mourning, he started to notice little, wonderful things in life. The little happiness brings him joy everyday. It makes him warm.

Kai is warm. Jaein is cold. What will happen to them when they meet again after ten years of lost connection?