One Piece's "Wano" arc is one of the densest and most complex in the series' history. The culmination of a saga that began hundreds of chapters ago in the "Punk Hazard" arc, Wano sees the Straw Hat Pirates and their allies clash with the forces of one of the Four Emperors of the Sea, Kaido, to end his tyrannical reign over the titular country.
"Wano" is an arc hundreds of chapters in the making -- the country was first mentioned all the way back in Chapter 450 -- and it currently stands as the longest in the series. As such, there are many plot threads in which to become tangled. Here's a refresher for those who need reminding how Luffy and friends got here and what they're fighting for.
As shown in a flashback set twenty years before the events of Wano, Kaido teamed up with Kurozumi Orochi to fill the power vacuum left behind by Wano's heir Kozuki Oden during his travels with Whitebeard. While Orochi officially holds the title of Shogun in the present, it's clear that Kaido is the one pulling the strings, with Orochi nothing more than a puppet for the Emperor of the Sea.
The prospect of an alliance to take Kaido down was first raised by Trafalgar Law during the "Punk Hazard" arc. Along the way, the alliance against Kaido has grown beyond just the Straw Hats and the Heart Pirates, and for good reason. The despotic King of the Beasts has made a lot of enemies on his path to start history's greatest war and die a glorious death, with numerous factions eager to hasten his journey to the grave.
Chief among them are the Nine Red Scabbards -- the former vassals of Kozuki Oden. Luffy and co. first met one of the Scabbards during Punk Hazard, encountering their leader, "Foxfire" Kin'emon, held captive on the island. Along with Oden's son Momonosuke, Kin'emon accompanies the Straw Hats on their mission to defeat Kaido, reuniting with his fellow Scabbards along the way.
Kozuki Oden and his followers form the emotional center of the "Wano" arc. Oden has quickly become one of the most popular characters in the series, both in and out of universe. His carefree nature belies a deep love for his country and its citizens, paralleling Luffy's similarly brash yet compassionate personality.
The Scabbards' devotion to Oden likewise mirrors the Straw Hats' dedication to Luffy, making it all the more devastating when Oden was killed in cold blood by Kaido twenty years ago. The faith the citizens of Wano have in Oden is still felt in the present, with the return of the Kozukis representing hope following the despair of Kaido's tyrannical reign.
Even Kaido's son, the fan-favorite Yamato, has been spurred to join the series' heroes, in part thanks to the folktales surrounding Oden. Oden's effect on the citizens of Wano ties into some of the broader themes of One Piece about how people live on through stories and even the long-dead can inspire hope for future generations. It's now up to Oden's son Momonosuke to carry on his father's will and help the dawn rise on Wano once more.
Aside from Kin'emon, other notable Scabbards include Nekomamushi and Inuarashi, the two leaders of the Mink Tribe, a clan of anthropomorphic mammals. The Minks share a deep connection with the Kozuki Family, and the "Zou" arc saw their home ravaged by one of Kaido's subordinates, Jack the Drought, giving them plenty of personal stake in the fight against Kaido.
The Straw Hats' friendship with the Scabbards already provides plenty of incentive for them to take down Kaido, but there's even more at play for the protagonists. During their stay in Wano, Luffy and co. have befriended many of the locals, such as the young kunoichi Tama and the late Shimotsuki Yasuie of Ebisu Town.
Through these allies, the heroes discover the plight of the people of Wano. Its citizens are starved and brutalized by Kaido's forces, with Ebisu Town in particular being a victim of unique cruelty. The Straw Hats oppose injustice wherever they go, and Wano is no exception, making it clear exactly what's on the line should they fail in the battle against Kaido.
However, taking down Kaido won't just liberate Wano -- it will also bring Luffy one step closer in his journey to become Pirate King. Kaido being another contender to that throne would be reason enough for the Straw Hats to want to take him down, but the revelation that Kaido possesses one of the four Road Poneglyphs, the guideposts to the One Piece itself, means that the King of the Beasts has to go down if Luffy wants to claim the legendary treasure for himself.
Both the anime and the manga are presently depicting the alliance's raid on Onigashima, Kaido's base of operations. While the manga is further ahead than the animated adaptation, neither version of the story is close to concluding the arc. There are many plot threads yet to be resolved, such as Kaido's temporary alliance with fellow Emperor Big Mom, the fruits of Luffy's training with Yakuza boss Hyogoro and the presence of CP0 on Wano.
While the heroes seem likely to triumph against Kaido, how the rest of the arc will play out otherwise is anyone's guess. Oda remains as unpredictable an author as ever, and One Piece is all the better for it. Wano has been the series' biggest and most epic saga yet, and no doubt its climax will suitably match the rest of the arc in that department.