This is my favorite book ever.
So basically, JAck and Ellie go to the nurse at the same time (They don't know each other yet) and the lights go out. The nurse lady starts chanting something, and when the lights go back on, POOF, they switched bodies. We experience Ellie (Her mind in Jack's body) standing up to Jack's father, and Jack (In Ellie's body) going through "Girl Problems". In the end, Their parents, (Jack's dad & Ellie's mom) get married, and they become siblings.
Like I said, best book ever, would read for the 3073rd time. :)