The adventures of Monkey D. Luffy and his pirate crew in order to find the greatest treasure ever left by the legendary Pirate, Gold Roger. The famous mystery treasure named "One Piece".
The crew disguises the Going Merry as a Marine ship so they can get their treasure back. With the gold, they decide they’ll hire a shipwright.
Usopp splits up the crew into two teams: the gold retrieval team and the waver retrieval team. Inspector Shepherd decides to catch Luffy on his own.
On their way to "The Palace of the Sea God" Robin's group meets an entranced Zoro. It looks like Zoro wants to fight so Luffy urges the others to keep going without him. They apprehend the kid but the whistle was what was really stealing their memories. Zoro returns, still entranced, but Luffy's nowhere to be seen.
Paulie makes his debut when he retrieves Usopp's stolen money as he runs from debt collectors. Kalifa shows Luffy, Nami, and Usopp around Dock No.1 and tells them how Iceburg founded the Galley La Company. Kaku returns from the Going Merry and tells them that their keel is irrepairably damaged.
Chopper escapes the burning building with Paulie and Iceburg on his back and collapses outside. Iceburg makes it clear to Nami that Robin wanted to recover the blueprints and to do that, she wanted her companions to leave the island without her. Nami rushes to tell Chopper so they can go looking for the rest of the crew. Elsewhere, the foremen let go of the Going Merry so she would be destroyed at sea.
Luffy easily gets past the second gate, while the Franky Family and the Galley-La foremen, following their initial plan, break the first gate's defenses. All goes smoothly at first, but by the time the Franky Family's main force reaches the second gate, the marines have called for help. The giants Oimo and Kāshī, the island's front gate gatekeepers, take up their posts. Overmatched in strength and size, the Franky Family send their best fighters, the Kairiki Destroyers. However, even these overly large humans can do little against the giants' power and are quickly defeated. On the other side of the gate, Luffy faces the collective might of Enies Lobby's 10,000 men.
Oimo explains to Sogeking about how he and Kāshī came to support the marines—the marines told them that they arrested their bosses. The story reminds Sogeking of the giants Dorry and Broggy, whom the Straw Hats met shortly after entering the Grand Line. Sodom the king bull incurs severe damage and urges the Straw Hats riding him to continue without him. Quickly, they board Sodom's brother Gomorrah and continue on. Atop of the courthouse, the fight between Blueno and Luffy begins. Luffy throws the first punch and seems far less inferior than during their last encounter.
The Straw Hats storm the courthouse, where the remaining members of the Franky Family fight the three-headed judge Baskerville, and, aided by Yokozuna, block the building's entrance to cut off reinforcements. Above them, Luffy's Gear Second is put into action against Bluno, while his crew fight their way upstairs. On the main road, the giants have defeated the Law's Watch Dog Unit, and with Sogeking and the Galley-La foremen are heading for the plaza. Luffy defeats Blueno, steps past his unconscious body, and cries out for Robin to hear that he has arrived.
Twenty years into the past, Robin is growing up on the island of Ohara. Her aunt, with whom she lives while her mother is away doing research, treats her poorly. Other children ostracize her. Studying in the world's greatest library, the Tree of Omniscience, in the hopes of being allowed to help the local archaeologists in their forbidden research, Robin earns the rank of a scholar in archaeology at age eight. However, when she announces her intention to help, her request is denied because of her youth. Disappointed at being rejected again, she walks along the shore, where she stumbles on Jaguar D. Saulo, a giant and deserted vice-admiral of the navy. The two become friends and Saulo teaches her to laugh.
Chopper's breather concludes as Kumadori frees himself from the fridge, apparently after he finished eating its contents. At first, he is fat, but he digests all the food immediately and is slim again. Fascinated, Chopper asks how he did it. Kumadori willingly explains. Afterwards, the two resume their fight where it had stopped. Chopper quickly realizes that he cannot break through Kumadori's body-hardening technique without the effect of his Rumble Ball drug. Having used a first while rescuing Nami, he dares not use a second out of fear of its side effects. Nevertheless, he takes it. Chopper charges repeatedly, but his transformations are erratic and difficult to control. By the time he assumes the correct form, he is too weak and is struck down. Left with one choice, he steps into the forbidden realm of taking a third Rumble Ball.
After a grueling struggle, Luffy finally defeats the Don with a red-hot Bazooka. With a little help from Lil, the two stolen flags are finally recovered, and the crew manages to get it back on the ship before Luffy returns. The defeated Accinos sail away with the Don in tow and finally have a belated birthday celebration, while Albelle makes up with her sister. The Phoenix Pirates go to battle with Canpaccino and the remaining grunts, Puzzle promising to meet the Straw Hats again in the New World.
Usopp and the others return to the Sunny in search of Luffy, Zoro, and Sanji. When they arrive, they discover the trio unconscious. After waking them up, Usopp and Franky each share what they have discovered. But, Franky has a shocking story about Brook's promise to his nakama. Meanwhile, Nami's wedding with Absalom draws near.
After learning that Brook's nakama is Laboon the whale, Luffy is dead-set on recruiting him. Before they prepare to take down Thriller Bark and regain their shadows, Sanji tells Chopper, Robin, and Franky the story of Laboon and the Twin Capes.
With their goals in mind, Luffy, Sanji, Robin, Usopp, and Chopper go after Nami and Moria! Meanwhile, Franky and Zoro head toward Brook. However, Hildon tells the others of the Straw Hat's plan.
After Lina reveals that she has searched all over the ship, Luffy decides to destroy the ship in order to find Sayo. The Straw Hats manage to fight off Doran's men, but Doran unveils a large cannon and aims it at Sayo. Sayo tells Lina that their father truly cared for them and believed that his research would bear fruit, and left in order to protect them from his pursuers. The Straw Hats manage to rescue Sayo and destroy Spa Island, and notice that the X on Lina's father's map pointed to an X-shaped underwater volcano opening beneath the island. Several days later, Sayo and Lina complete the gem and send it to the Straw Hats, but as candy rain falls on the Sunny, Luffy accidentally loses it.
After reading an account of the various adventures that the Straw Hats have had, Brook resolves to be as much use as he can to prove his worth, but his well-intentioned efforts only manage to annoy or get in the way of the others. Brook then reveals his insecurites to Robin, saying that he'll leave the crew if he proves to be only a burden. Robin tells him that she used to feel similarly, until the Straw Hats came to save her at Enies Lobby, and she had finally found her place. Encouraged, Brook later reflects on how he has found his place after decades of being alone, and resolves to do his best so that he can have stories to tell of his own.
After traversing the treacherous Serpent Currents, the Straw Hats finally arrive at the Red Line, signifying that they've sailed halfway around the world and that their voyage is half over. While the crew reflects and takes a well-deserved rest, a sea monster attacks. Luffy easily dispatches it, forcing it to spit up a talking starfish and a genuine Mermaid onto the Sunny's deck.
Duval bears a grudge against one of the Straw Hats, but it is not known which one. When the Straw Hats reach the hideout, they discover that Hacchin is none other than the octopus Fishman Hatchan, formerly of Arlong's crew. Although reluctant to save him with this new knowledge, Keimi convinces the crew to go through with it. However, everything was a trap set by Duval, and the Straw Hats are ambushed by the Flying Fish Riders.
While the battle between the Straw Hats and the Flying Fish Riders rages on, Zoro frees Hatchan from his cage, and the Fishman in turn saves him from an attack, revealing that he feels bad about their battle at Arlong Park. Meanwhile, Luffy crashes into Duval's hut and catches a glimpse of his unmasked face before being chased back outside. Once outside, Duval reveals that Sanji is the Straw Hat he wants to kill. After Luffy knocks his helmet off, the reason for his hatred is laid bare: Marines and bounty hunters have been relentlessly pursuing him because he looks exactly like the poorly-drawn visage on Sanji's wanted poster!
After the surprising reveal, Duval tells his story of living a peaceful life as a mafioso until Marines and bounty hunters came after him for Sanji's bounty, mistaking him for the Straw Hat, casing him to flee and wear a helmet to hide his face. The Flying Fish Riders attempt to drown Sanji by dragging him underwater, but the cook is saved by the intervention of Keimi. Duval then tries to sink the Thousand Sunny by having his men drop a giant anchor on it, but Franky reveals the Chicken Voyage reverse function to dodge it, and then the Gaon Cannon to wipe out most of the Riders. Enraged, Duval orders his pet bison Motobaro to gore Luffy, but Luffy stops the beast by unknowingly using haki, the beast goes wild and then faints, much to everyone's surprise. Sick of everything, Sanji takes things into his own hands, and sends Duval flying into his hut with a faceful of kicks.
The crew splits up in order to explore the archipelago. Hatchan, Keimi, and Paggag lead Luffy, Chopper, and Brook to find the mechanic Hatchan spoke of. In town, they witness a disturbance caused by a man claiming to be an escaped slave yelling for help in getting an iron collar off. To the Straw Hats' horror, the collar explodes, severly injuring the man. Two people wearing gaudy clothing and bubble helmets, Saint Roswald and his daughter Saint Shalulia arrive and callously shoots his body twice before leaving him for the Marines to pick up. Paggag later explains that they were part of the Celestial Dragons, descendants of the founders of the World Government, and that they have the power to summon one of the three Admirals if they so please.
After fighting their way through bounty hunters, Luffy's group arrives at a bar owned by a woman named Shakuyaku, an old friend of Hatchan's. The mechanic, Rayleigh, is not there, so Shakky suggests that they go searching for him at the Sabaody Amusement Park, but not before warning them to be careful, as right now there are nine other pirates on the Archipelago with bounties over 100,000,000 Beli. In addition to Luffy and Zoro, they are: Capone Gang Bege, Jewelry Bonney, Basil Hawkins, Eustass Kid, Scratchmen Apoo, Dies Drake, Urouge, Killer, and Trafalgar Law. Together, the group is known as the Eleven Supernovas.
Duval and his crew arrive on the archipelago and start gathering the Straw Hats up to bring them to the Human Auction House, where the kidnappers have brought Keimi. Robin tells Nami why the Fishmen and Merpeople are not welcome on the island as in the past they had been discriminated against and classified as just fish. At the auction house itself await Saint Roswald and Saint Shalulia, as well as Trafalgar Law, Eustass Kid, and their crews. Backstage are Keimi and the other human merchandise, including the coating mechanic, Silvers Rayleigh, former first mate of Gold Roger himself.
Luffy wakes up inside a cage, surrounded by the curious Amazons. Marguerite gives him a new pair of cloths to wear, but then the other Amazons decide to finish him off, as they detest men. They attack Luffy with cannon-strength archery, but he grabs Marguerite and manages to escape into the jungle, where he gets his Vivre Card back from her. Although initially distrustful, Marguerite's curiosity gets the better of her and learns that he's a pirate. She reveals that Amazon Lily's empress, Boa Hancock, is in command of an all-female pirate crew, which, unbeknownst to the two, is on its way back to the island.
Luffy attempts to make a raft to leave the island, but utterly fails and has to be saved from drowning by Marguerite. She determines that she's getting too attached to him, and resorts to attacking him with her super-powered archery, which she reveals is the result of Ambition, the same power that Rayleigh used to knock out everyone at Sabaody. Luffy is then driven off by the arrival of the other Amazons. Offshore, the ship of Boa Hancock and her two younger sisters is met by a Marine battleship helmed by Vice-Admiral Momonga, who is there to take Hancock, the final Shichibukai to Mariejois in preperation for the battle with Whitebeard. Hancock uses the Swoon Swoon Fruit (Mero Mero no Mi) to petrify the lovestruck Marines, and tells Momonga that she refuses the order.