This is a really, really general question. It would be really hard for one of us to explain the plot of the fifth book as there are so many twists and turns, but there are a couple of options for you.
a) READ THE BOOK. This is the best, really. Then YOU know what is going on and it isn't through some interprative lens of someone else.
b) I'm almost loathe to provide you with this link, but eh. Here it is. Here are Mugglenet's sypnopses of all six books.
From what you've said, you seem like a person who's watched the movies rather than read the books. I urge you to actually read them, as they're full of so much more detail, humor, suspense, plot... then the movies could ever be. Agfain, I urge: READ THE BOOK! If you have, and you've just forgotten, READ IT AGAIN!