Warship Island Arc

As the earliest of the anime's invented storylines, this arc was written strictly for television and traces to no chapter Eiichiro Oda ever drew. It sits at the tail of the East Blue Saga, beginning right as the crew sails out of Loguetown with Luffy carrying his newly minted thirty-million bounty, and wrapping up just before they climb Reverse Mountain into the Grand Line. Being the show's first original arc, the staff reached higher than most later filler would, lacing their invented material through several genuinely canonical scenes. That reach also bred a string of contradictions with facts Oda would later lock in, from a Marine rank that does not exist in canon to the presence of a dragon, a creature the manga would long treat as fiction.

A girl named Apis escapes a vessel of Marine Branch 8, capsizing it as she flees and drifting off on a tiny boat. The branch's obese, immobile captain Nelson Royal sends the claw-handed mercenary Eric the Whirlwind to retrieve her, believing she holds the key to a youth-restoring elixir. Ringed by seagulls and mistaken for a shoal of fish, Apis is scooped up onto the Going Merry, and the crew slowly earns her confidence. They learn she can talk to animals through a Devil Fruit gift and has befriended a Sennenryu, a thousand-year dragon long thought vanished, which she hopes to bring home.

Two problems block the way: a Marine fleet wants the beast for a lucrative medicine and has hired Eric, who secretly means to keep the dragon for himself, while nobody knows where its homeland actually is. Along the route the crew blunders into the Calm Belt, meets its enormous Sea Kings, and slips away before pressing on to Warship Island. From there the story becomes the crew's effort to reunite the Sennenryu with its birthplace, keep Apis happy, and stay clear of Eric's designs.

Its closing installment turns back to the Reverse Mountain material, adding a scene in which Eric stows away on the Merry for one last strike before Nami sends him tumbling into the water. The arc left a faint echo in later filler, as the villain Tatsu of the Ocean's Dream story yearns to become a Sennenryu, and Luffy's power to understand animals here oddly foreshadowed his later canonical knack for hearing Sea Kings. The 4Kids Entertainment dub dropped the whole arc, ironically tracking the manga by sailing the crew directly from Loguetown into the Grand Line. No other filler arc has ever been turned into a novel, yet this one was, as 2001's Legend of the Sennenryu, and the sole one Oda is known to have praised in print, while series director Konosuke Uda later noted the Sennenryu idea had once been floated for a theatrical film.