One Piece is about the main character "Luffy", who made a promise to a man named "Shanks" that he'll become pirate king. Luffy ate an "Devil Fruit" which man him into a "Rubber Man" letting him to stretch his body.
Along the journey, he searches for the ultimate treasure, "One Piece" with the help of his friends.
There's a lot more to the plot, I just told you the main portion.