The series focuses on Monkey D. Luffy, a young man who, inspired by his childhood idol and powerful pirate "Red Haired" Shanks, sets off on a journey from the East Blue Sea to find the famed treasure One Piece and proclaim himself the King of the Pirates. In an effort to organize his own crew, the Straw Hat Pirates (麦わら海賊団篇 Mugiwara Kaizoku-dan), Luffy rescues and befriends a swordsman named Roronoa Zoro, and they head off in search of the One Piece. They are joined in their journey by Nami, a navigator and thief; Usopp, a sniper and a liar; and Vinsmoke Sanji, a womanizing chef who is also a prince from a family of Royal Assassins. They acquire a ship named the Going Merry and engage in confrontations with notorious pirates of the East Blue. As Luffy and his crew set out on their adventures, others join the crew later in the series, including the doctor and anthropomorphized reindeer Tony Tony Chopper, an archaeologist and former assassin Nico Robin, a cyborg shipwright Franky, and skeletal musician Brook, and they also acquire a new ship named Thousand Sunny.
The world of One Piece is populated by humans and many other races such as "fishmen" (a race of fish/human hybrids, similar to mermen and mermaids), dwarves, Minkmen (a race of humanoid animals), and giants. It is covered by two vast oceans, which are divided by a massive mountain range called the Red Line (赤い土の大陸 レッドライン Reddo Rain), which is also the only continent in the world. The Grand Line (偉大なる航路 グランドライン Gurando Rain), a sea that runs perpendicular to the Red Line, further divides them into four seas: North Blue (北の海 ノースブルー Nōsu Burū), East Blue (東の海 イーストブルー Īsuto Burū), West Blue (西の海 ウェストブルー Uesuto Burū) and South Blue (南の海 サウスブルー Sausu Burū). Surrounding the Grand Line are two regions called Calm Belts (凪の帯 カームベルト kāmu beruto), similar to horse latitudes, which experience almost no wind or ocean currents and are the breeding ground for huge sea creatures called sea kings (海王類 kaiōrui, lit. "sea kings"). Because of this, the calm belts are very effective barriers for those trying to enter the Grand Line. However, navy ships, members of an intergovernmental organization known as the World Government, are able to use a sea-prism stone (海楼石 kairōseki) to mask their presence from the sea kings and can simply pass through the calm belts. All other ships are forced to take a more dangerous route, going through a mountain at the first intersection of the Grand Line and the Red Line, a canal system known as Reverse Mountain (リヴァース・マウンテン Rivāsu Maunten). Sea water from each of the four seas runs up that mountain and merges at the top to flow down a fifth canal and into the first half of the Grand Line. The second half of the Grand Line, beyond the second intersection with the Red Line, is known as the New World (新世界 Shin Sekai). The currents and weather of the Grand Line's open sea are extremely unpredictable, whereas in the vicinity of islands the climate is stable. The magnetic fields within the Grand Line cause normal compasses to malfunction, making it even more difficult to navigate, and instead a special compass called a Log Pose (記録指針 ログポース Rogu Pōsu) must be used. The Log Pose functions by locking on to one island's magnetic field and then locking on to another island's magnetic field. The time for it to set depends on the island. This process can be bypassed by obtaining an Eternal Pose (永久指針 エターナルポース Etānaru Pōsu), a Log Pose variation that is permanently set to a specific island and never changes. The world of One Piece includes anachronisms, such as the Transponder Snails (電伝虫 Den-Den Mushi), snail-like animals that can be attached to electric equipment and function as rotary phones, fax machines, surveillance cameras, and similar devices. Dials (貝 ダイアル daiaru), the shells of certain sky-dwelling animals, can be used to store kinetic energy, wind, sound, images, heat, and the like and have various applications.
A Devil Fruit (悪魔の実 Akuma no Mi) is a type of fruit that, when eaten, confers a power on the eater. A person may only eat one Devil Fruit during their lifetime. There are three categories of Devil Fruits; Zoan (動物系 ゾオン Zoon) fruits allow the user to fully or partially transform into a specific animal. As well as real-life animals, some Zoan fruits allow the user to transform into mythical creatures. Logia (自然系 ロギア Rogia) fruits give control over, and allow the user "to change their living body structure into the powers of nature". Paramecia (超人系 パラミシア Paramishia) is a category of fruits that give the user superhuman abilities. Devil Fruits are said to be incarnations of the sea devil himself, and as a result, Devil Fruit users cannot swim in sea water, as "they are hated by the sea". Sea-prism stone also has this effect. When even partially submerged in sea water, they lose all their strength and coordination, although some abilities remain. For example, Luffy is still able to stretch after being totally submerged. Moving water, such as rain or waves, does not have this effect. When a Devil Fruit user dies, the powers will be reincarnated into a new Devil Fruit. For unknown reasons, Devil Fruit users can't eat a second Devil Fruit because it would cause their bodies to burst, though Blackbeard manage to get a second Devil Fruit power through a unknown method. Devil Fruit powers can be in a stage called "Awakened", where the user can turn anything around them, besides their own bodies, into what their Devil Fruit power is.