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The Ruluka Island Arc, also referred to as the Rainbow Mist Arc, is the fourth filler arc in the One Piece anime. This is also the last arc dubbed by 4Kids.
The arc starts off with the Going Merry being chased by a small fleet of Marine ships led by Major Rapanui Pasqua and Isoka. Luffy and his crew barely escape when the Marine's Major accidentally sinks one of his own ships. The crew then make their way towards an island named Ruluka that is ruled by a dictator, who is actually an old pirate, that is over-taxing the people in order to build the Rainbow Tower. The island also has a scientist named Henzo who is researching something called the 'Rainbow Mist'. It is revealed that he is sponsored by the unscrupulous Wetton, mayor of the city and a former pirate. Eventually the Going Merry, with all but Sanji, Nami, and Chopper aboard, gets caught in the Rainbow Mist. Within the Mist, the crew discovers a group of children who have been trapped in the mist for many years. Eventually they find a way out, but when the Straw Hats reemerge, they cannot find the children. The marines come to "arrest" them, which are apparently the kids, but say there is no room on their ship due to Wetton and his gang being locked up. The Straw Hats leave the island, and then a ship falls from the sky.
Unlike in the manga, where the Going Merry was heavily damaged at the end of the Alabasta Arc by Hina's attack, in the anime, this arc depicts how the Going Merry began to severely fall apart, seeing it get blasted out of Ape's Concert with too much force for the ship to handle, causing the figurehead to break off, and have one of the masts snap in two and collapse on a railing, tearing it apart as well. Then, the ship is patched up in Ruluka just before heading out.
The Merry being shot out of Ape's Concert with an explosive blast bears an uncanny, yet unrelated resemblance to the Coup de Burst function its successor, the Thousand Sunny, would have, a ship built strong enough to handle the shock from such force and weaponize it.