The Reverse Mountain Arc, also known as Laboon Arc, is an extremely short yet significant story arc of One Piece, where the main characters would comprise the Straw Hat Pirates: Monkey D. Luffy, Roronoa Zoro, Nami, Usopp, and Sanji. There will also be an introduction of Laboon, the huge whale, and Crocus, the keeper of the lighthouse. This arc forms the first part of the Alabasta Saga in both the manga and anime.
This arc takes place as the Straw Hats try to enter the Grand Line. The main reason is their journey to sail through perilous Reverse Mountain, which is a special geographical feature that serves as the main entrance to the Grand Line. It has upward-flowing currents, so ships can climb the mountain and, from there, enter the Grand Line from any one of its four Blues.
The plot of the Reverse Mountain Arc begins with the approach of the Straw Hat Pirates to the entrance of the Grand Line. As they head toward it, they realize that they are quite close to Reverse Mountain. The challenge that befalls them next is guiding their ship, the Going Merry, through hazardous currents and up the slopes of the mountain. This requires navigation and teamwork, outlining how cohesive the crew can now be and how well each member has developed his skill. It was a perilous, amazing initiation into the unpredictability of the Grand Line, showing many adventures that would follow.
Now finally at the top of the mountain, the Straw Hats encounter an unexpected obstacle: Laboon, the enormous whale blocking their path at the top of the mountain. Laboon's presence introduces a poignant subplot to the arc, as the crew learns from Crocus, a lighthouse keeper who has taken care of Laboon for decades about his backstory. It turns out that Laboon had been abandoned by the pirate crew that vowed to return after sailing around the Grand Line but never did so. This story speaks to the Straw Hats, but most especially to Luffy, to whom Laboon displays unbreakable loyalty and the wounds of promises.
One of the pivotal moments of the arc is Luffy's meeting with Laboon. Wanting to stop Laboon from killing himself by ramming into the Red Line over and over (the giant continent that outlines the world), Luffy decides to engage in a "fight" with the whale. This is where the unconventional battle comes to an end: Luffy declares it a draw and promises Laboon that they will come back for a match once they have conquered the Grand Line. What seals this promise is that Luffy paints a rough version of the Straw Hat Pirates' jolly roger on Laboon's head, which symbolically and emotionally binds the crew to the whale.
Although the Reverse Mountain Arc is rather short, it has a few vital purposes with respect to the entire One Piece narrative. It finally sets the Straw Hats into the Grand Line and marks the beginning of all their biggest and most fantastic adventures. Their meeting with Laboon and Crocus reveals information regarding the nature of the Grand Line and what still lies ahead. The introduction of the Log Pose—this special compass for navigation in the Grand Line—is crucial for further travels of the crew.
Thus, at the end of the Reverse Mountain Arc, the Straw Hat Pirates set off to the Grand Line with renewed determination and understanding of the world they are entering. Luffy's promise to Laboon adds an extra dimension to their incentives, hence personal goals are intermingled with the commitment to return. On the day of parting, the crew had learned something, gained equipment, and got a feel for the highly unpredictable nature of the Grand Line. The Reverse Mountain Arc thus acts as a transition bridge from the early East Blue adventures of the Straw Hats to the still larger and more challenging odyssey waiting for them in the Grand Line.