On Ruluka Island, some of the Straw Hats make enemies of the Mayor and his men, who have placed high taxes on everything. Robin, Usopp, and Luffy meet Professor Henzo, who's doing research on the island's rainbow mist. When some of the mist shows up, Henzo borrows the Going Merry to go explore it.

The inside of the Rainbow Mist full of wrecked ships and treasure, and is guarded by five kids. The kids are actually friends of Henzo's that got lost in the mist over 50 years ago.

Henzo, his friends, and the others try to find a way out of the ship graveyard. Meanwhile, the Mayor and his men let the information about the Rainbow Mist slip to the remaining Straw Hats still on Ruluka.

Luffy accidentally propels himself and Rapanui to the end of the mist. Nami appears in the ship graveyard with a rowboat full of treasure and a string connected to the real world, which are both stolen by Wetton's subordinate. The Mayor uses the Rainbow Tower to make a bridge to the Rainbow Mist.

The Going Merry escapes from the Rainbow Mist, getting badly damaged on the way. The Mayor gets arrested by a bunch of powerful marines that turn out to be older versions of Henzo's friends, who were thrown 50 years into the past by the Rainbow Mist's collapse.