The Whole Cake Island Saga is one of Eiichiro Oda's One Piece manga and anime major story arcs and a part of the Yonko Saga. The arc includes chapters from about the end of volume 82 to the beginning of volume 90 of the manga series and episodes 783-877 in its anime adaptation, which makes it a large part of the 19th season.
This saga focuses on the Straw Hat Pirates' operation of rescuing Sanji, their crew member, from an arranged political marriage with Charlotte Pudding, daughter of Big Mom (one of the Four Emperors, Yonko), set up by his family, the Vinsmokes. The operation takes them to Whole Cake Island, which is the stronghold of Big Mom Pirates located in the New World part of the Grand Line.
A unique setting in this One Piece world is Whole Cake Island itself, part of Totto Land, a land entirely of edible substances. It represents Oda's creativeness in world-building, which resonates in the aesthetic feel of candy and the usage of food in every possible aspect of design. This setting provides a very stark contrast to the much darker themes explored during the arc, such as forced marriage, family abuse, and the ruthless nature of Yonko politics.
The arc introduces major characters to the One Piece universe. Finally, Charlotte Linlin, who is wide and famously known as Big Mom, was an antagonist that has been shown to be powerful and had stolen lifespans of people, able to put souls into an object with her powers from Soul-Soul Fruit. Her enormous family—85 children, each with different abilities and personalities—fleshes out the Big Mom Pirates group.
This arc goes way back into Sanji's past and his roots as a prince of the Germa Kingdom, including the abuses he had to take from his family. It fleshes out much about Sanji as a character, detailing to a great degree why he despises the name of the Vinsmoke family to such a great degree, and also the details of his brothers: Ichiji, Niji, and Yonji, who have been genetically enhanced with special abilities, and his father Judge, who is a scientific genius behind their altered genes.
Probably among the most prominent things concerning the Whole Cake Island arc is the revelation of more advanced Haki forms. Thus, the fight of Luffy against Charlotte Katakuri, Big Mom's strongest son, represents the inclusion of the Future Sight advanced application of Observation Haki. This fight is perhaps one of the most intense and well-choreographed in this series because it literally pushes the bar up for Luffy and offers a significant power increase to the main protagonist.
The arc has equally great character development from the other Straw Hat members. Brook's solo infiltration into Big Mom's treasure room all by himself really went well, as he even got a copy of the Road Poneglyph despite meeting the Yonko himself. Nami shows a strategic use of Zeus, who is a thundercloud with Big Mom's soul infused, to show how much she's grown as a fighter and a navigator in dangerous situations.
This is where politics in the territory of a Yonko comes in: the Whole Cake Island arc. How complex this system of alliances, betrayals, and power balances within the family and crew of Big Mom is portrayed very nicely. It also touches on wider issues of discrimination, as seen in the case of Totto Land, against non-humans—thereby deepening the social critique within One Piece.
This conclusion opens out to the entire One Piece world. From Katakuri's defeat to crashing Big Mom's tea party, Luffy's activities during the Arc earn him the media nickname "Fifth Emperor," an indication of his rising status in the pirate world. This development seals in the stage for future conflicts and alliances in the ongoing narrative.