The Straw Hat Pirates, who are the main characters in this Skypiea Arc, include Monkey D. Luffy, Roronoa Zoro, Nami, Usopp, Sanji, Tony Tony Chopper, and Nico Robin. Some of the other well-known characters are Enel and Gan Fall, Skypiea's former God, Conis, Pagaya, Wiper, and the Shandians, which consist of Aisa and Kalgara.
The arc takes place in Skypiea, one of the sky islands in the White-White Sea, featuring its cloud-based environment, unique flora and fauna, and advanced technology run through Dials. The prime essence of this arc is that the Straw Hats want to explore Sky Island after learning about it through the Jaya Arc, to quench their curiosity and adventurous spirit.
The plot of the Skypiea Arc revolves around the spectacular scene of the rise of the Straw Hat Pirates into the sky island using the Knock-Up Stream, where they find a special civilization involving Skypiea: Skypieans and Shandians, two groups at war with each other. The crew soon becomes embroiled in the complex political and historical tensions on this island, which focuses on the ownership of Upper Yard, a part of Jaya that was sent into the sky 400 years ago.
As the Straw Hats explore Skypiea, they learn of Dials, shell-based devices with various powers that seem to be integral to Sky Island technology. They also found out about the existence of Enel, the self-proclaimed God of Skypiea, who possesses the Goro Goro no Mi, a Logia-type Devil Fruit enabling him with lightning-based abilities. Enel's tyranny and his plan to destroy Skypiea in order to reach a legendary golden city called Shandora will be the core conflict of this arc.
The history of Skypiea is told, from the tale of Montblanc Noland, the explorer who, 400 years ago, came upon Jaya and discovered the City of Gold—their relation to the Shandians. Setting it in a historical perspective makes the current conflict all that more deep, along with the inherited will and pursuit of truth in One Piece.
Further into the arc, an intense three-way battle royale known as the Survival Game involves the Straw Hats fighting against Enel's priests, Shandian warriors, and finally, Enel himself. Each Straw Hat member turns out to face some challenge unique to all of them, each displaying their strengths and growth: most notably, Luffy, whose rubber body would become Enel's natural counter to the Lightning powers, pitting them against an epic battle.
The aforementioned arc finally culminates in the battle between Luffy and Enel, transpiring on the legendary golden bell of Shandora. This struggle will decide not only the fate of Skypiea but also sound the golden bell so that those below might hear it and prove Sky Island's existence to fulfill a promise that dates back 400 years.
Throughout, it contains themes of religious fanaticism, abuse of power, and the importance of history and cultural heritage. The concepts/objects that will show their roles in future arcs first revealed in this arc are, for instance, the Dials and the Poneglyph with their information about ancient weapons.
The Skypiea Arc concludes with Luffy's triumph over Enel, the golden bell, and settling the Skypieans' conflict with the Shandians. Peace was restored to Skypiea as Gan Fall resumes being God and begins his leadership. The Straw Hats set off to sea once again with new knowledge, experiences, and treasures: gold from the ancient city, and Dials that will prove useful in future adventures.
This arc is important in the One Piece narrative for several reasons. It does more world-building by introducing sky islands, which further shows that the Grand Line consists of diverse and fantastical locations. The arc also further deepens the series' exploration into history and how it can affect the present, something often showing up as a running theme throughout One Piece. It also has quite a lot of development for the Straw Hats, particularly Nami and Robin, and it introduces several themes and technologies that become relevant later.