There is this legendary pirate who conquered every sea and amassed a giant treasure and left it on an uncharted island. He was captured by the world government and publicly executed, but with his final words he revealed the existence of his treasure to the world, which launched a wave of new pirates on a search for the hidden island. It follows the pirate Strawhat Luffy as he collects a crew and works his way to finding the treasure and becoming the new pirate king while fighting the marines and rival pirates. The world is complex, but that's a basic overview.

I can't really think of anything I could compare it too. Like I said, the first few introductory arcs when the first few crew members come together are pretty meh, but it accelerates quickly, and the point it is at now every episode is epic as shit. The series just starts out a little silly.