Quick disclaimer, when this was written One Piece had reached Chapter 675. Since this takes place after the events of One Piece, I'm gonna make up a lot of story details which might not match up with what Oda writes in later chapters. So if, somehow, this story is still going on in a couple of years, don't complain if that stuff doesn't match up.
Many years ago, there was a man named Gol D. Roger. King of the Pirates. The man who brought together a crew of legends and managed to do the impossible. On the day he was executed, he changed the world and the world entered the First Age of the Pirates.
Several years later, his successor, Monkey D. Luffy set out on his journey to become the Second Pirate King, bringing together an even stronger crew and even surpassing the exploits of his predecessor. Those who saw his final epic battle with Fleet Admiral Sakazuki, at the Edges of the Earth, had the sight permanently ingrained into their mind. He became the first man to find Gol D. Roger's legendary treasure, the One Piece, almost toppling the World Government in the process.
Two Years later, and Monkey D. Luffy stood on an execution platform facing his end. The executioners standing over him had been given one simple instruction.
'Under no circumstances let the Second Pirate King speak.'
However, Luffy's crew momentarily intervened, giving the pirate the chance to speak to the world.
Here are the last recorded words of Monkey D. Luffy
Shishishi! I bet you all want to know about One Piece, right?
That treasure was the most amazing thing I've ever found!
But it's too much for just me to take! It's still there! You're welcome to it!
But only if you think you can surpass me and Roger!
The cheer that went up that day was even greater than the World Government feared. Long retired pirates leapt to their boats, with their children and their grandchildren, all in search of the elusive treasure and the chance to surpass the two legends. If Gol D. Roger started the Great Pirate Age, Monkey D. Luffy started the Greater Pirate Age. And the new pirates who rose that day quickly became legends. This story is about of one of the greatest of those pirates, Ushio D. Neptune, 'Emperor of the Pirates'. A name that made others shiver with fear and excitement and a crew that may have surpassed even Monkey D. Luffy's. But when he began his journey...
Ushio D. Neptune looked at the pool of water slowly rising at his feet, and wondered how he'd gotten himself into this mess.
Okay, so when he'd made the small hole in the boat, in an attempt to juggle a pack of kitchen knives, he hadn't really done much to fix it. In fact, his plan at the time had been 'ignore it and hope it goes away'. Which hadn't really worked out. In his defence, at the time, the hole had been very small and how was he supposed to know that it'd slowly get bigger? Unfortunately, there was still quite a distance before he'd hit land and, considering the ship was already half-submerged, he had a feeling he wouldn't make it.
Suddenly, he spotted a small crate of explosives, bobbing up and down in his cabin. And he had a brilliant idea...