A young Korean girl moves with her parents from Korea to "heaven," i.e. America. But once she gets there everything is definitely not heavenly. In fact, life is pretty much continuously difficult for Young Ju and her parents.
First they have to live with Young Ju's aunt and uncle, which isn't all that comfortable (you try living with your extended family), and then they move into what can only be described as the ghetto.
From there things only get harder. Young Ju's little brother is born, which basically means that Young Ju goes from baby of the family to total afterthought… and then the real stuff really starts to happen: her father, Apa, gets increasingly mean and violent.
Sure he shows signs of being a jerk before this point, but with all the pressures of maintaining a young family in a new country (where he can hardly speak the language) on the income from two super-demeaning, low-end jobs, Apa just isn't having a good time in America. So he takes it all out (and we do mean all) on his wife and kids.
The rest of the book is all about Young Ju dealing with her dad as well as the Koreanness of her family. She grows up and goes to high school, where she has a (white) American best friend who hardly knows anything about her Korean family (or even where she lives)—that's because Young Ju keeps her family's stuff so secret that she might as well be a CIA agent undercover.
She does all of this while pulling straight A's and getting into college. And as for her dad… well, let's just say she gives him the boot with a little help from the po-po and her changed mother.
Does that mean all her insecurities and fears are gone? No way, but by the end of the novel she's definitely on her way—literally—out of her world and into a better, new one. And so are her mother and brother. Apa, on the other hand, goes all the way back to Korea.