Karan (Saif Ali Khan) is a cartoonist. His characters Hum and Tum reflect the strange love-hate relationship between men and women. Rhea (Rani Mukerji) is a sensitive, self-confident girl. Initially, they have very little in common. As life would have it, their paths keep crossing and over the course of a decade, their relationship evolves, from hate to mutual respect, friendship and finally love.