At the heart of this story is the romance between Yukino Miyazawa and Soichiro Arima. Yukino is an honors student, and the envy of her classmates and appears to be the perfect model student, but this is just a facade that she puts on because she loves to be praised and is a self proclaimed "Queen of Vanity". When she starts high school, she is shocked that someone else beat her entrance exam score and became the class representative, that person being Arima.
This spawns a rivalry (at least in Yukino's perspective) between the two, and its during this time that Arima discovers that the Yukino at school is just a front, and to keep him quiet, he makes Yukino his slave. They become friends, and eventually, Arima confesses his feelings for Yukino to her, and they begin dating.
The story has a fairly large cast of characters, and throughout the story, they all get their time in the spot light, and the manga covers subjects like bullying and peer pressure, academic stress, complex family issues like child abuse and abandonment, marrying young and unexpected pregnancies.