Spa Island Arc

The Spa Island Arc is the ninth anime-only filler arc. It takes place after the Thriller Bark Arc, and chronicles the Straw Hat's stay at Spa Island, where they run into Foxy, Porche , and Hamburg. It is also the first filler arc with Brook.

Summary
The mini-arc begins with the Straw Hats already on the island, an artificial place that houses many recreations the crew enjoy individually as explained by Doran, the owner of the island, to Nami and Robin. Meanwhile Luffy, Chopper, and Usopp goof around in the wading pools and water slides, attracting the attention of a young girl who enjoys their antics. She introduces herself as Lina and her pet sea raccoon, Munki, who can shape-shift (albeit always small in size). Lina also points out her sister Sayo, who is busy looking over a notebook. Soon Nami, Sanji, and Robin come over to meet the girls though they can't help but be curious of the notebook Sayo closely guards. Feeling a little uncomfortable, Sayo decides to leave the island with Lina and Munki in tow.

Before they can leave, Foxy, along his loyal crewmates, Porche and Hamburg suddenly appear much to the Straw Hats' surprise (though Luffy takes awhile to remember who he is). While still miffed at his defeat since their last meeting, Foxy's only concern is Sayo's notebook. He reveals a new mecha, a mirror covered image of his head which his crew quickly enter into and spin around. Foxy then shoots his Noro Noro Beam out from under it causing the beam to reflect off the mirrors and shoot everywhere. Both the Straw Hats and the sisters are hit and slowed, allowing Foxy to easily take the notebook from them. Foxy then snaps his fingers and the floor underneath the group is slid out from under them, which will cause them to fall into the ocean once the thirty second time limit is up.

Franky and Brook suddenly come into the area having just left the spas though wondering what the rest of the crew is doing floating in the air. As the Noro Noro Beam finally wears off, Luffy shouts for Franky to beat Foxy just and he and the others plunge toward the ocean. Franky obliges and blasts Foxy with his Weapons Left causing the residents on the island to panic and quickly flee the area. Meanwhile Luffy manages to save the sisters as well as himself while Robin catches the other falling cremmates. Foxy is angered that Luffy managed to make more crewmates since their last encounter and runs back to his mecha. However, Hamburg closes the door on it too hard breaking the mirrors over it. Luffy then hits Foxy's group with his Gomu Gomu no Bazooka, knocking the three into the ocean.

After the commotion finally dies down, Nami eventually points out the notebook Sayo is holding who finally decides to explain the sister's story. The two are the daughters of a scientist who believed he could make a genuine gem. However the village they lived in found the idea foolish and would often ridicule the scientist. Some of the village children even playing pranks on him like painting a gold horned beetle (which was said to bring misfortune to a village if it was found) and laughing as the scientist freaked out after finding it. Still he always laughed these off, looking on the bright side of everything as he loved the village dearly since it is where his wife was born (who apparently died, she was only shown in a photograph). Then one day their father headed off on a long expedition taking Munki with him and leaving the sisters by themselves.

After about a year, Munki suddenly returned with the notebook containing their father's notes and a letter asking the sisters to finish his research, the scientist apparently having died. Lina however is bitter because she thinks her father abandoned Sayo and her, seeing no point in the research since they can't decipher the notebook. But after hearing the story, Luffy thinks otherwise citing their father sounded like a good man. Upon hearing that, Sayo starts to gather up the courage to ask for the crew's help in the research but a trapdoor suddenly opens under her and she falls through. Unknown to the group, Doran had been eavesdropping on their conversation with his Den Den Mushis over the island and scheming to take treasure for himself, having been the one that employed the Foxy Pirates and set them on the sisters. The group panic over what to do until Luffy makes the suggestion of simply destroying the island much to Doran's panic.

The crew (which includes Zoro who now noticed the other tourists leaving) muddles over it but come to conclusion that its the best idea since they will find Sayo faster that way. They proceed to the wreck the place, taking out the island's security that comes after them in the process. Doran eventually stops the bunch by showing them Sayo tied to a stake and a gigantic cannon aimed at her. To show he was serious, he fires a warning shot that barely misses Sayo. Doran then demands the notebook for Sayo's life. Lina prepares to give it up but Sayo yells out not to, explaining their father never abandoned them and tell of the day of Lina's birthday when he wasn't able to come back home in time.

After a few days passed he eventually returned with some items needed for his research. He noticed the cake however, realized his folly, and ate it all resulting in a stomach ache. When Sayo and he were alone, he explained his regret for not making it back in time and decided to dedicate his gem research to making one for Lina. The reason he never came back home when he went out on his last expedition and sent his notebook to the sisters was because the Foxy Pirates were tailing him and he didn't want to put his daughters in danger. Upon realizing the truth, Lina has a change of heart and yells out her refusal to Doran to relinquish the notebook.

The Straw Hats then proceed to stop Doran and rescue Sayo. Luffy, with Lina and Munki holding onto him, jumps into the air, uses Gear 3rd, and preform his Gomu Gomu no Giganto Axe move splitting the island in half and sinking it (much to the annoyance of his crew since they have to jump onto the debris). However Luffy forgets about his shrinkage side effect from using Gear 3 causing Lina and himself to go plunging into the ocean. Luckily Sanji jumps in after them. As he pulls the two up, Lina notices an x-shaped crack on the ocean floor leading to an underwater volcano. When she reaches the surface, she explains this find to the everyone. Sayo checks the notebook over and sees it was the x-mark they were looking for, which was right under Spa Island (and in effect under Doran's nose). The heat from the lava soon makes steam form around the group and the reflection from the sun causes a rainbow effect. The sisters realize the steam is the final ingredient to make the gem. Doran and Foxy's crew see this phenomenon as well. Upon realizing that that was the treasure all along, Doran allows Foxy to "keep it" and swims away with Foxy's crew chasing him, angry that they won't get any material treasure for their trouble.

A few days later, the Straw Hats' continue their journey when Munki flies in and delivers a letter to the crew from his owners. Sayo and Lina succeeded in making the rainbow gem and are living happily with their father who returned, alive and well, just in time to see the creation of the gem. Also, the sisters sent the rainbow gem to the Straw Hats as thanks for their help. The crew gather around to admire the jewel when it suddenly starts raining candy on them. In the midst of this, the gem gets knocked out of Luffy's hand prompting Nami to frantically order everyone to search for it on the deck.

Trivia

 * Spa Island is the second island that Luffy has personally destroyed, the first being Clockwork Island. Also, both are non-canon.