The tale begins with the execution of Gold Roger, the legendary King of the Pirates, who sparks a global "Great Pirate Era" by announcing his treasure, the One Piece, is hidden at the end of the Grand Line. Twenty-two years later, we meet Monkey D. Luffy, a relentlessly optimistic dreamer who ate a "Devil Fruit" as a child, granting his body the properties of rubber at the cost of his ability to swim. After his rowboat sinks, Luffy finds himself as a stowaway on the ship of the cruel Lady Alvida. There he meets Koby, a timid cabin boy forced into servitude who secretly dreams of joining the Marines. Luffy defeats Alvida with a powerful "Gum-Gum Pistol" and encourages Koby to follow his heart, marking the first of many dreams the young captain will ignite.

Seeking a map to the Grand Line, the duo travels to Shells Town, where Luffy encounters the stoic pirate hunter Roronoa Zoro, who is imprisoned by the corrupt Marine Captain Morgan. While infiltrating the base, Luffy runs into Nami, a clever thief who hates pirates but is after the same map. The trio reluctantly works together to overthrow Captain Morgan, liberating the town and reclaiming Zoro's three swords. While Koby stays behind to enlist in the Marines, Luffy, Zoro, and Nami set sail, though the latter two insist they are not yet a "crew".

Their journey takes a dark turn when they are captured by Buggy the Clown, a pirate who ate the Chop-Chop Fruit, allowing him to split his body into floating pieces. After surviving a circus-themed torture session and crating Buggy's limbs to defeat him, the group frees a ransacked town and heads to Syrup Village in search of a sturdier ship. There they meet Usopp, a lovable liar known for shouting "The pirates are coming!" to the annoyance of his neighbors. They discover Usopp's best friend, the sickly heiress Kaya, is being slowly poisoned by her butler, Klahadore, who is actually the notorious Captain Kuro. Luffy and his companions defeat Kuro's Black Cat Pirates, and in gratitude, Kaya gifts them the ship *Going Merry*. Usopp, seeking to become a brave warrior of the sea, officially joins as the crew’s marksman.

While sailing, the crew is intercepted by Marine Vice Admiral Garp, who is shockingly revealed to be Luffy’s grandfather. After a narrow escape, they arrive at the Baratie, a magnificent floating restaurant. There they meet Sanji, a suave chef with a moral code never to let anyone go hungry. Tensions erupt when Garp hires Dracule Mihawk, the world’s greatest swordsman, to capture Luffy. Zoro, driven by a childhood vow to a fallen friend, challenges Mihawk to a duel. He is handily defeated, with Mihawk breaking his swords but sparing his life out of respect for his resolve, telling him to grow stronger.

The peace of the restaurant is shattered when the Fishman pirate Arlong arrives, and Nami reveals she has been working for him all along to steal the Grand Line map. Luffy refuses to believe she is evil and follows her to Conomi Island. We learn Nami’s harrowing history: Arlong killed her adoptive mother, Belle-mere, and forced Nami to map the seas for him. She had been secretly hoarding 100 million Berrys to buy her village's freedom, but Arlong double-crosses her by having a corrupt Marine confiscate her savings. In a moment of absolute despair, Nami stabs her Arlong tattoo and begs Luffy for help. Luffy simply places his hat on her head and declares, "Of course I will".

The Straw Hats launch an all-out assault on Arlong Park. While Sanji and Zoro handle the Fishmen grunts, Luffy takes on Arlong himself. Enraged by the maps Nami was forced to draw, Luffy destroys the entire building on top of Arlong, ending his reign of terror. In the aftermath, Garp arrives to arrest his grandson, but after a fierce physical test, he relents, realizing he cannot break Luffy’s spirit. Luffy is awarded his first "Wanted" poster with a 30 million Berry bounty. The season concludes with the five core Straw Hats placing their feet on a barrel, each vowing to fulfill their individual dreams as they set a course for the Grand Line.