The series begins with the execution of Gol D. Roger, a man known as the King of the Pirates (海賊王 Kaizokuō?). Just before his death, Roger announces that his treasure, the One Piece (ひとつなぎの大秘宝 (ワンピース) Wan Pīsu?), will be available to anyone who finds it, causing the Great Pirate Era (大海賊時代 Dai Kaizoku Jidai?) to begin. As a result, countless pirates set out to the Grand Line to look for the treasure.
Twenty-two years have passed since Roger's execution, and the story focuses on Monkey D. Luffy, a young boy who, after inadvertently eating a devil fruit as a child, has become a rubber man capable of stretching any part of his body in exchange for his ability to swim. Inspired by his childhood idol and powerful pirate Red Haired Shanks, Luffy sets off on a journey from the East Blue Sea to find Roger's legendary treasure and become King of the Pirates. In an attempt to organize his own crew, the Straw Hat Pirates (麦わら海賊団篇 Mugiwara Kaizoku-dan?), he befriends a swordsman named Roronoa Zoro and together, they sail off to find the One Piece. They soon meet Nami, a navigator and thief; Usopp, a sniper and a liar; and Sanji, a womanizing chef; leading to confrontations with Buggy the Clown, Captain Kuro and Don Krieg. Later, Luffy encounters Arlong, a fishman and member of the former Sun Pirates who thinks that fishmen are superior to humans. After Luffy defeats Arlong, Nami officially joins Luffy's crew and the Navy places a bounty on Luffy's head. After making their way through the Grand Line, the group meets Nefeltari Vivi, a princess who wants to help save her country, the Alabasta Kingdom, from the crime syndicate Baroque Works. They later befriend the doctor and anthropomorphized reindeer Tony Tony Chopper while in Drum Island.
The Straw Hat Pirates make their way to Alabasta, leading to battles with Baroque Works and their leader, Sir Crocodile. Luffy eventually defeats Crocodile and liberates Alabasta. Soon after, Nico Robin, an archaeologist and former member of Baroque Works joins Luffy's crew. They soon meet Blackbeard, whose dream is also to become Pirate King. After going up to the floating island of Skypiea, the crew gets involved in a war between the Skypieans and the Shandorians, leading to a confrontation against the island's ruler Eneru, who has the power of lightning. Luffy defeats Eneru to save Skypiea and end the war. The crew soon meets the navy admiral Aokiji, who reveals that Robin was involved in searching for Poneglyphs, which are stones with markings left by an ancient civilization to reveal the missing 100 years of history that the World Government had erased. The group goes to Water 7, confronting the cyborg shipwright Franky and find that their ship, Going Merry, has sailed for the last time and must be dismantled. This leading to Usopp's temporary departure from the Straw Hats, only to return to fight as the Sniper Hero Sogeking. However, Cipher Pol No. 9, the World Government's intelligence agency captures Robin and Franky for information regarding the Poneglyphs and the ancient weapons that may come from them. Franky break free from the World Government after burning his blueprints for one of the weapons and teams up with the Straw Hat Pirates to declare war on the World Government, resulting in battles between CP9 and its minions. The final long battle with CP9 ends when the crew saves Robin, Franky building their new ship, Thousand Sunny, and joins their crew. Soon after, the crew helps a musician skeleton named Brook, who ate the Revive-Revive fruit which allowed him to come back to life, only to find his body had decayed. Together they battle in Thriller Bark and find his shadow, which has been stolen by Gecko Moriah.
The crew later prepares to sail off to the New World, the second half of the Grand Line, after arriving at the Sabaody Archipelago. While there, they befriend Silvers Rayleigh (or Dark Knight Rayleigh), a former member of Roger's pirate crew who plans to coat their ship so they can travel underwater. The crew eventually gets separated during a battle with Bartholomew Kuma, a huge cyborg under the control of the Navy, at the Sabaody Archipelago, with Luffy being sent to the all-female island Amazon Lily. When he learns that his older brother Portgas D. Ace has been detained at the government prison Impel Down, Luffy goes there and liberates several people from the prison,including the fishman Jimbei and some old enemies. Luffy soon learns that Portgas D. Ace is at Marineford to be executed. However, war breaks out between the Navy and a group of pirates led by the legendary pirate, Whitebeard. In the ensuing chaos, Whitebeard and Ace are killed. At Rayleigh's suggestion, Luffy has the Straw Hats undergo rigorous training regimens, some under the tutelage of prominent figures, changing their meeting date from 2 months to 2 years.
Two years later, the crew regroups at Sabaody Archipelago and journeys to Fishman Island to enter the New World. During this time, a group of fishman pirates appear, seeking supremacy against humans, and hold a coup d'état to decide the fate of the island. However, after Nami forgives Jimbei for releasing Arlong into the East Blue, the Straw Hats defeat the fishmen pirates, saving the island. The Straw Hats leave Fishman Island and finally reach the New World, but not before starting a feud with one of the four strongest pirates in the New World, Big Mom. Entering the half-burning and half-freezing island, Punk Hazard, the crew encounters an old friend and one of the new Warlords, Trafalgar Law, and they form an alliance to take down the four strongest pirates in the New World. Trafalgar Law was also the pirate to patch up Luffy after the war at Marineford. After forming their new alliance, they were drawn into a fierce battle against Caesar Clown, who had destroyed Punk Hazard 4 years prior with a massive bomb. Caesar plans is to recreate his bomb and make even stronger to make sure no living being survives. Caesar was also working for Doflamingo, a dangerous Warlord with an even more dangerous power to control other people's actions. With Doflamingo's orders, Caesar successfully recreated one of Vegapunk's experiments, creating Zoan type Devil Fruit in project SMILE. After Caesar's defeat, the alliance goes to Dressrosa, a kingdom ruled by Donquixote Doflamingo, in an attempt to destroy the SMILE factory as the next step to take down another one of the four strongest pirates, Kaido. Upon arriving in Dressrosa, Luffy becomes distracted trying to win back his brother Ace's devil fruit from the Corrida Coliseum, while Law goes with Robin and Usopp to give Caesar back to Doflamingo, only to find out that Doflamingo faked his resignation as a warlord for Law to give back Caesar, but unfortunately bringing attention to one of the admirals, Fujitora.