The Return to Sabaody Arc includes the following characters: Straw Hat Pirates (Monkey D. Luffy, Roronoa Zoro, Nami, Usopp, Sanji, Tony Tony Chopper, Nico Robin, Franky, and Brook), Silvers Rayleigh, Shakky, and various Marine officers.
The most prominent reason behind this arc has to do with simply the reunion of the Straw Hat Pirates after a period of two years since their separation and training. Sabaody Archipelago, the place where the events of this arc take place, is noticeably marked with mangrove trees and technology manipulated by bubbles; on the other hand, it serves as an entrance to the New World.
The plot of the Return to Sabaody Arc commences with the eventual return of the Straw Hat Pirates to Sabaody Archipelago, which began two years ago. With every character possessing a new ability and new looks, it is patently clear that each one of them grew during the time skip. This saga focuses on their reunion and what lies ahead for them on their journey to the New World.
On arrival, many issues strike them, and various abilities unfold of the new power. For instance, Luffy's potential to control his Haki is unleashed, and other supporting crew members, such as Nami, Usopp, and Chopper, show their main areas of expertise. Other characters, such as Rayleigh and Shakky, also returned in this supporting character arc to provide informative resources and even skills for the sake of the crew.
Much of the arc focuses on keeping the Straw Hats from the detection of the Marines and other dangers on the archipelago. This includes confrontations with impersonator pirates who merely use the Straw Hats' names to gain some renown, which adds a mistaken identity element and some humor to the story. Key to the plot is them trying to get back to their ship, the Thousand Sunny, which has been kept safe in their absence by Rayleigh and his Rosy Life Riders.
The arc reaches its climax when the Straw Hats finally face off against the Marine forces to thwart their departures from Sabaody, showing off this newly found strength within the crew and their teamwork—just how much they had grown as individuals and as a unit during their separation.
This arc is filled with so many references to what has happened in the last two years or even mentions of the Marineford War and all that happened afterwards. These reflections form the main context for what the motivations of the crew were like and how the world changed during their absence.
At the end of the Return to Sabaody Arc, the Straw Hat Pirates reunite into the Thousand Sunny and finally leave Sabaody Archipelago. As much as they are on their way to Fishman Island, which is the first destination in the New World, it feels exciting and highly anticipated for the future adventures.
This arc was one of the most critical plot twists in the entire One Piece storyline. It connected very well with the pre- and post-time skip parts of the story, re-introducing major characters with their stronger abilities and acting as a launching point into New World for the challenges to come. The arc also closed out the events leading up to their separation and built up anticipation for future conflicts.