The Giant Mechanical Soldier of Karakuri Castle

One Piece The Movie: The Giant Mechanical Soldier of Karakuri Castle (ONE PIECE THE MOVIE カラクリ城のメカ巨兵,) is the seventh movie in the anime and manga series One Piece. This is set before the events in the Water 7 arc.

Synopsis
While searching a wrecked ship for treasure, the Straw Hat Pirates come across a treasure chest. After Usopp picked the lock and was opening the chest (to classical music,) they find an old woman in the trunk (no treasure.) In the process of bringing her back to her home island, they learn about a Golden Crown there. Though they come in conflict with the woman's son, who is lord of the island, they decide to stay in search of this treasure, using an old song they heard on the island for clues.

Plot
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 "Kame 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. 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 handglider 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.

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 it's 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 two snake's 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 Japanese Crested 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 it's 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 battlesuit (which Luffy calls "cool"), which Luffy destroys, Ratchet quickly comes back with another robot attached to a boat that can fight on both water and land (which Luffy call "uncool"), unfortunately it's a disaster when it couldn't even reach Luffy, Ratchet then decides to get his ultimate battlesuit. 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 battlesuit. During the battle, Luffy accidentally activates Gear 2nd and easily destroys it (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 goin 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 (Gum-Gum Spinning Axe) 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.

Trivia

 * This movie, while technically not considered canon, features a possible explanation for the origin of Luffy's Gear 2nd which he unwittingly discovers during his fight with Ratchet. However, this makes it so that the only possible timeframe for this movie to take place is sometime near the encounter with Aokiji and the Water 7 arc, since it was stated that Luffy did not think of Gear 2nd until he met Aokiji, although he could've meant that he didn't think of that event until Aokiji.
 * 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, and often bounce at the slightest movement or are excited, accompanied with a rather humourous "bounce" sound effect. This is mostly because of Eisaku Inoue (the animation director of this movie) who is known for his good depiction of the female characters in the anime. 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 Ohtani, was not used in this movie as she was on maternity leave, and also, Chopper's voice gets deeper during his Heavy point (something that just appears in the 4Kids dub).
 * Usopp can be seen working on the Perfect Clima-Tact during the opening credits.