Help with movie title and stuff?!?

Okay so I remember hearing about this movie when it first came out but I cant remember the title or anything. It has Leonardo Dicacaprio in it and the girl who played Rose in Titanic. And I found it last night on tv and only caught the middle half of it...what is the title and what exactly is the whole movie about? It seemed interesting but made little sense since I only caught a small part of it. And I didnt get too see the very end because of my brother :( help!!!