Now caught in a sea current, the crew resolves to navigate the Going Merry up through the waterway, and even though the steering rudder snapped, Luffy was quick enough to veer the ship on the right course, sending the ship rushing up the waterway and onto the right course down towards the Grand Line, only veiled by a blanket of mist. However, trouble suddenly arises as they face another obstacle in their path... a giant, scarred whale howling to the sky, with the crew in a tight spot due to the broken rudder.
Before they can crash into the whale, Luffy gets the idea to fire the front cannon at the whale to decrease their speed; though it isn't enough to keep the ship from ramming into the whale, only snapping the figurehead off. However, in a fit of rage after the destruction of his special seat, Luffy throws a punch directly at the whale's eye, prompting it to open its maw and swallow the Going Merry whole, with only Luffy narrowly managing to avoid being eaten. His attempts to get the whale to cough his crew up in vain, Luffy discovers a peculiar hatch on the whale's body moments before it submerges.
Inside of the whale, the rest of the crew come upon a peculiar sight; a blue sky and an island with a summer home before them. A giant squid appearing before them is suddenly killed by harpoons, shot out by an old man making his residence in the small village home. He introduces himself as Crocus, the lighthouse keeper of Reverse Mountain, and points out that the pirates were indeed swallowed by the whale. Suddenly, their surroundings start shaking violently, as Crocus laments that the whale is ramming into the Red Line. As he heads off to sedate the whale, the crew prepare to head off.
In the meantime, having found himself in metallic corridors, Luffy is hurled about once the whale's insides start shaking up, crashing straight into a mysterious duo and sending all three of them hurtling directly into the whale's stomach. After fishing the three up, the duo fires directly upon the whale's stomach, but Crocus saves him by taking the cannon shells himself, before Luffy promptly knocks the two of them out.
With that conflict solved, Crocus tells them more about the whale; named Laboon, he came from the West Blue along with a group of pirates about fifty years ago. While they had tried leaving him behind because of the danger the Grand Line posed, he had followed them into the great ocean. After befriending Crocus, the captain asked him to take care of Laboon for a few years, after which they would return.
After sailing out of Laboon and back into the actual sea, the Straw Hat Pirates toss the mysterious duo overboard. After introducing themselves as Mr. 9 and Miss Wednesday, they promptly swim off, leaving the others to ponder the ultimate fate of Laboon's old crew; Crocus believing that they fled the Grand Line via the Calm Belt after losing hope. He explains that while he did try to tell this truth to Laboon, he refused to listen, and began bellowing towards the mountain and slamming himself against the Red Line as a result. Due to this, Crocus remains adamant in remaining by Laboon's side, to ensure that the whale doesn't end up killing himself.
At that point, though, Luffy suddenly tears off the Going Merry's mast, and begins engaging Laboon in a seemingly random match, much to the shock of everyone else. After a vicious battle between the two of them, however, Luffy suddenly declares that the match is a tie, and that he'll definitely come back after their voyage around the world has finished, so he and Laboon can settle the score; giving Laboon with a new goal to wait for. To mark this promise, Luffy paints the Straw Hat Pirate mark on Laboon's head, covering up most of his scars.
At this point, Nami discovers that their compass has started going haywire, with Crocus explaining that with how the Grand Line is structured, ordinary compasses are useless in the Grand Line, and that they need a Log Pose to navigate from island to island, until eventually reaching the last island of the Grand Line; Raftel, supposedly where Gold Roger left the treasure One Piece. As it turns out, Luffy happened to procure a Log Pose after Mr. 9 and Miss Wednesday dropped it when they were tossed overboard, but when it's accidentally shattered thanks to Luffy and Sanji's antics, Crocus offers his own log pose as thanks for aiding Laboon.
Meanwhile, Mr. 9 and Miss Wednesday, while thinking of means to get back both their Log Pose and Laboon, are attacked by The Unluckies; an otter called Mr. 13 and a vulture called Miss Friday; sending them careening into the ocean where they pick themselves up by Reverse Mountain, where they resolve to make a deal with the Straw Hat Pirates. In exchange for leaving Laboon alone, the Straw Hats would take the two of them to their hometown; Whiskey Peak. Luffy agrees to this, and with the new companions at the ready, the Straw Hats bid Crocus and Laboon farewell as they set sail. As they sail off, Crocus comments that Luffy has a certain aura to him, as if he's talking to Gold Roger himself...
Laboon Arc has the following tropes
- Attack of the 50-Foot Whatever: Laboon initially.
- Berserk Button: Don't break Luffy's seat!
- Big Bad Duumvirate: Mr. 9 and Miss Wednesday.
- Bigger on the Inside: Subverted, when the Merry gets eaten. The crew find an oddly outdoors-like setting with a small island and house in the middle of it, confusing them till Crocus revealed they are inside the whale's stomach. He just built a small house-like ship and painted the stomach so he can have an atmosphere within Laboon. What's more there are even passageways in the stomach as well.
- Combat Medic: Crocus. He's spent two decades fixing Laboon up from the inside and sometimes even lives there. He, of course attacks any dangerous creatures Laboon may swallow up. The end also clues in on his role in relation to Gold Roger.
- Dub-Induced Plot Hole: The 4Kids dub skipped this arc and changed Laboon to an iceberg that Luffy blows up. Needless to say once the Thriller Bark Arc comes up, many folks were chuckling about the Hilarious in Hindsight. A good thing their dub ended when it did or they would've had a hard time explaining that.
- Eye Scream: Luffy punches Laboon in the eye for breaking his favourite seat (the figurehead).
- Foreshadowing: The whole tale of Laboon's backstory, which wouldn't be covered again until the Thriller Bark Arc.
- Leaning on the Fourth Wall/Running Gag: When the crew first meet Crocus, he somehow keeps doing dramatic close ups of himself. After the third one, the crew yell for him to knock it off.
- Metaphorically True: We learn that Crocus' explanation for the disappearance of Laboon's crew is technically right. They came down with a disease that ravaged the crew, with the captain and the infected members taking the ship into the Calm Belt to probably die. The remaining members, however, continued down the Grand Line until they were wiped out in the Florian Triangle, with Brook as the sole survivor.
- Mr. Exposition: Crocus, who explains how Log Pose and the Grand Line works.
- Pet Baby Wild Animal: Laboon, who was only a baby that followed a certain pirate crew to the Grand Line and stayed within the area in the hopes of them returning.
- The Promise: The crew that Laboon was with promised they would return someday after sailing the Grand Line, though they never returned. Luffy makes a new promise with Laboon in the form of a fight and that they will resolve it when they come back. For added insurance to keep Laboon from harming himself further. He paints the Straw Hat Jolly Roger on Laboon's face, claiming it'll wash off if he continues.
- Retired Badass: Crocus. His primary weapon are harpoons, and he is pretty strong.
- The Reveal:
- The final island on the Grand Line is named Raftel. It's mysterious to the point that only Rodger's crew have found it, and it is believed to be where One Piece is located.
- Crocus was a crewmate of Gold Rodger.
- Scars Are Forever: Laboon has some nasty ones on his face from ramming into the mountains so much.
- Shout-Out: Crocus's houseboat within Laboon looks an awful lot like Kame House from Dragon Ball.
- Smart Ball: Luffy, in a remarkably...unconventional manner, since he appears to be playing kick-ups with it. The Going Merry is sliding towards the Grand Line, but they're about to be smashed to bits by a giant whale serving as a blockade, so what does Luffy do? Fire a cannon at it! This slowed down the ship's speed to a snail's pace and the whale barely felt the cannon, leaving the Merry completely unharmed...until the figurehead breaks, and Luffy promptly drops it when his Berserk Button was hit.
- Swallowed Whole: What Laboon does to the Straw Hat Pirates...and a pair of bounty hunters.
- Tempting Fate: When the Straw Hats have just found the Log Pose, and Nami is reflecting on its importance:Nami: All right, Log Pose... I'll make sure to take good care of you. The fate of our journey is all up to you, after all.
- Villains Want Mercy: Mister Nine and Miss Wednesday after being defeated both want the Straw Hat Pirates to take them home to Whiskey Peak. The Straw Hats eventually decide to take them. Later subverted since neither of them are actually evil.
- Wham Line: "Maybe these are the pirates we've been waiting for. Don't you think so...Roger?"
- Womb Level
- World Building: A lot of it. In just five chapters, we learn about the Calm Belt, Log Posse's and the general structure of the Grand Line.
- The World Is Just Awesome: The Going Merry going up the Reverse Mountain and the view of the Grand Line at the top.