Having recovered a treasure chest from a sinking ship, the Straw Hat Pirates can not wait to see what riches await them inside. Instead of treasure, they pulled out an old lady. Anxious to get back home (and avoid Nami's wrath for having thrown out the treasure previously in the chest), the old lady offers a great treasure reward if the crew will return her to her home on Mecha Island. Her legend of the Golden Crown sounds too good to be true. Luffy smells adventure, so the crew has no choice. On an island full of amazing inventions, the crew will test their riddle-solving skills—and Luffy's instinct—to solve the mystery of Mecha Island and its treasure.
During a raging storm, the crew searches a wrecked pirate ship for any treasure only to find a single chest. On the Going Merry, they open it only to find an old woman who hid there to escape the storm. As they contemplate what to do with her, even suggesting to put her back in the box, the woman tells them about a legendary Golden Crown on her island that she will let them have if they return her there. Though the rest of the crew thinks this sounds suspicious, Luffy agrees. As it turns out, it is impossible to reach using a Log Pose, requiring the use of a "Turtle Pose", which uses a small turtle that always points to the island.
As they come up to the island, the crew hears a fisherman singing an old folk song about the Golden Crown known as the Yurari Uta. The woman explains that the island is known as Mecha Island and as the pirates get closer, they set off a defense system involving arrow-firing robots. When the island's leader, Ratchet, sees the old lady, his mother, on the ship, he launches a hand glider and retrieves her from the ship. Then, one of Ratchet's subordinates, General Maji, activates the robots, sending hundreds of arrows flying towards the ship, but Zoro deflects all of them. Maji then activates a giant fan to blow the ship away, but the arrow robots end up getting sucked into it. Though the metal parts being blown out of the fan wreck the Going Merry's main sail, the fan is destroyed while Maji gets his back broken by Robin's Clutch.
Due to this, the crew cannot leave the island, so they decide to stay and search for the Golden Crown. They realize that there may be hints in the song they heard earlier, which Robin had thankfully written down. The two possible clues they find in the song refer to "the white snake's eyes" and "the two moons". They manage to figure out that the white snake is referring to a rock shaped like a snake head, but cannot understand what the two moons refer to. Usopp suggests that maybe it is the moon and its reflection, but that is wrong too. As it turns out, Ratchet is also trying to figure out the same thing, but he decides to leave the Straw Hats alone for now, believing that it is impossible for them to solve the mystery. During a contest between him and Usopp, Luffy breaks off one of the snake rock's eye, and Robin realizes that they may have misinterpreted the lyrics of the song: Tsuki can either mean "moon" or "strike" depending on the spelling, so the song could actually mean to hit the snake's two eyes. Luffy breaks off the other eye, revealing an entrance into the island. When Ratchet hears about this from Maji, who was watching the crew, he freaks out for a bit but eventually calms down and decides to use them to get the treasure for himself. Maji then invites the crew to a party at Ratchet's castle.
At the party, Ratchet asks for the Straw Hats to help him find the treasure. Though the rest of the crew finds this suspicious, Luffy agrees to help. During these events, Chopper keeps hearing a voice calling out. The next day, the crew along with Ratchet and his friends enter the cave. They pass through a narrow passage and fall down a waterfall before crashing their ship on land. Luffy comes across a beautiful lake surrounded by animal statues with light shining through the top, and Robin finds a pillar with the final part of the song written on it. At first, they think it means to say that it is time to drink up the lake, but they soon realize that it actually means that the Ibis can drink up the lake. Luffy unintentionally knocks down a giant statue of this bird into the lake, and all of the water drains away. At the bottom of the lake is a passage, which Robin, Chopper, and Sanji go through while the others stay to investigate the newly exposed surroundings.
Robin's group comes out at the top of the mountain and they discover that it is covered with fossilized barnacles. They find a shell-shaped temple, which they enter to find that the ceiling is littered with gems. Nami uses Thunderbolt Tempo on a lance found in the lake and the entire island starts to fall apart, revealing its true form: a giant turtle. As it turns out, it was the turtle that was calling Chopper before. Thus, the song actually serves as instructions to awaken the turtle island, which Ratchet reveals he knew this entire time, though he points out that he still has not figured out what the Golden Crown is referring to so there may still be a treasure.
At this point, Ratchet reveals his intentions: he will use the turtle to take over the world. His group manages to escape from the pirates and activates his giant mechanical castle. The Straw Hats gather together and break into the castle, but Chopper goes off on his own when the turtle starts speaking again. The remaining six are caught in a trap only to be released by Ratchet's butler. He and Ratchet's mother beg the crew to stop Ratchet, and he uses the castle to take control of the turtle. At this point, Chopper learns that the turtle is to be awakened every 1000 years to lay its eggs. The turtle island starts moving, and when Usopp notices this, he starts to run back outside to make sure the Going Merry is alright.
As Luffy runs on ahead, Zoro and Nami get separated from Sanji and Robin, landing in a roller coaster cart. As they travel along the track, getting farther away from the castle, they are attacked by Maji in a robotic cart. Meanwhile, Sanji and Robin are attacked by Ratchet's other henchman Honki in a mechanic suit. At the top of the fortress, Ratchet faces Luffy personally in another battle suit (which Luffy calls "cool"). After Luffy destroys it, Ratchet quickly comes back with another robot attached to a boat that can fight on both water and land (which Luffy calls "uncool"). Unfortunately it is a disaster when it could not even reach Luffy, so Ratchet then decides to get his ultimate battle suit. Zoro and Sanji easily defeat their foes and Nami manages to get off the track, where she meets up with Usopp and Chopper. Using the waver, which she hid earlier, they rush to the Going Merry to get it back. Luffy runs into Ratchet once again and battles his greatest robot battle suit. During the battle, Luffy accidentally activates Gear Second and easily destroys the robot (though technically the robot was described as being "far from complete" so it leaves speculation on how powerful it would be if it was finished), wondering about what he did afterwards. Everyone in the castle gathers at the wreckage.
Nami's group, who had retrieved the ship, meet up with everyone and tell them that the turtle is going to crash into a nearby island. Using a combination of attacks from Usopp, Sanji, and Zoro, Luffy executes an attack called Gomu Gomu no Kaiten Ono on the castle. Luffy breaks the castle apart, releasing the turtle. The turtle stops and successfully lays its eggs. As the crew start to leave, one of the eggs hatched. Luffy sees this and realizes that the Golden Crown is referring to the broken turtle egg shells, which are literally made of gold. They take one of the shells with them, but it ends up sinking into the water. Meanwhile, Ratchet is spanked by his mother for trying to take over the world.
|Mayumi Tanaka||Monkey D. Luffy|
|Kazuya Nakai||Roronoa Zoro|
|Kazue Ikura||Tony Tony Chopper|
|Yuriko Yamaguchi||Nico Robin|
|Goro Inagaki||Doctor Ratchet|
|Kōji Katō||General Maji|
|Keiichi Yamamoto||Colonel Honki|
- The trailer to the movie featured several scenes which did not make it to the final production, mostly due to deadlines. Most fans did not seem to notice, so there were not many complaints. A few of the said 'missing scenes' include Luffy fighting against a bipedal dinosaur-like robot and Sanji/Zoro's fights in daylight. The main reason fans were even aware of these scenes is that they were included in pre-release trailers and promos.
- This movie is notable for the obvious breast enhancements of Robin and Nami, which are larger, rounder, shinier, and often jiggle at the slightest movement or are excited, accompanied with a rather humorous "jiggle" sound effect. This is mostly because of Eisaku Inoue (the animation director of this movie). Inoue is also the animation director of the eighth movie Episode of Arabasta: The Desert Princess and the Pirates where the breast enhancements are seen again.
- The original VA for Chopper, Ikue Otani, was not used in this movie as she was on maternity leave.
- Usopp can be seen working on the Perfect Clima-Tact during the opening credits.
- This is the second movie with a mechanic-theme island (the first was Clockwork Island Adventure).
- This is also the second time where an island turns out to be a giant animal: a turtle in this movie, and a clam in the second episode special.
- This movie provides an explanation as to how Luffy developed his Gear Second technique. The doping process is trigged by accident when his legs buckle under the pressure of the Super Kougame-kun's leg and pump more blood through his body, giving him a brief boost in strength. This is also the first time Gear Second was seen animated.
- One Piece "Karakuri Castle's Mecha Giant Soldier" Site Promotional site for Movie 7.