One Piece is set in a fictional ocean-bound world similar to Earth, where pirates rampage wildly in an era of freedom and adventure known as the "Great Age of Piracy". This epoch was inaugurated with the public execution of Gold Roger, the Pirate King. With his dying breath, Roger announced that the world was free to seek the wealth he had accumulated during his lifetime, provided it could be found in "that" place (Grand Line). In the English adaptation, Roger is also credited with explaining that he "left everything [he] owned in one piece", hence the eponymous label for his glorious treasure.
Twenty two years after Roger's execution, interest in One Piece has dwindled; many have dismissed obtaining it as an impossibility, and some question whether or not it exists. Although pirates are still a paramount threat to the lives of civilians, the Navy has become more effective at halting pirate raids in the four Blue Seas. However, this laconic change of mood has not deterred the adventurous spirit of the Straw Hat Pirates, who travel to the Grand Line (where the spirit of the "Great Age of Piracy" still roars strong) in a hunt for Roger's treasure. Although the Straw Hats frequently find themselves at odds with other pirates, they are still deemed threatening by the World Government and Navy, and three of their number have been assigned high bounties.
The main characters of One Piece are Monkey D. Luffy and his crew, the Straw Hat Pirates. Along the way, they meet several other people, most of them are other pirate crews, while others are in the Navy, whose job it is to arrest these pirates.