Percy Jackson: The Sea Of Monsters is based on Rick Rodrian's novel, and explains greek mythology by narrating the story of a demigod living in modern USA.
And in this new opus, Percy and his friends are going on a quest to find the Golden Fleece in the sea of monsters. They need to save Camp Half-Blood (the only place safe for demi-gods like them) and Percy wants to change his destiny, no matter what The Oracle says.
I love the fact that the author insists on Percy's ability to act against the prophety, often using the following phrase
"I make my own destiny".
The movie is very well-adapted from the book and is a great way to learn about myths and legends.