The Thriller Bark Arc includes the following characters: Straw Hat Pirates (Monkey D. Luffy, Roronoa Zoro, Nami, Usopp, Sanji, Tony Tony Chopper, Nico Robin, and Franky) Key antagonists include Gecko Moria, one of the Seven Warlords of the Sea, and his subordinates known as the Mysterious Four: Absalom, Doctor Hogback, and Perona. Other significant characters introduced are Brook, a living skeleton, and Oars, a giant zombie.
The arc primarily centers on the encounter of the Straw Hats with a gigantic ship masquerading as an island: Thriller Bark, controlled by Gecko Moria. This characteristically gothic and haunted arc is full of zombies, ghosts, and other supernatural elements.
The story of the Thriller Bark Arc opens when the Straw Hats enter the Florian Triangle, a mysterious part of the sea, where they meet Brook, a skeleton with an Afro who is one of the central figures in this arc. The crew finally reaches Thriller Bark and learns that Gecko Moria has been using the powers of the Shadow-Shadow Fruit to steal the shadows of living things and insert them into corpses to raise an army of zombies. Shortly after entering through the gates, the Straw Hats find themselves separated, facing many hindrances in the creepy land known as Thriller Bark.
It is eventually revealed that Gecko Moria plans to create the ultimate subordinate, a giant zombie named Oars, with Luffy's shadow. This leads to a variety of confrontations between the Straw Hats and Moria's minions: fights against zombies he had created, and against the Mysterious Four. Notable conflicts include Zoro's fight against the zombie swordsman Ryuma, in which he manages to capture the legendary sword Shusui.
It tackles themes of identity and what a real person is, with the concept of shadows representing a person's soul and the horrors of having an identity stolen. A backstory of Brook is added, detailing his past association with the Rumbar Pirates and the promise he makes to return to the waiting whale, Laboon, at the very entrance to the Grand Line. This adds an emotional tinge to the arc and connects it well into the bigger picture of One Piece.
As the situation escalates, the Straw Hats have to rush against the oncoming daylight to retrieve their kidnapped shadows before sunrise, since their bodies would disintegrate in the sun without a shadow. Another climax of this arc involves the great fight against Empowered Oars and Gecko Moria, which needs the effort of the entire crew and temporary allies.
The arc reaches its peak with Luffy pushing himself to new limits in his fight against Moria, showcasing a new technique called Gear Second and Gear Third. Right after the conclusion of this battle, there is a surprising appearance by Bartholomew Kuma, another Warlord; he brings about a tense standoff in which Zoro is willing to sacrifice himself so that Luffy can retreat with the rest of the crew.
The Thriller Bark Arc finally reaches an end with Gecko Moria defeated and the shadows he had stolen revitalized. Brook also joins as the musician of the Straw Hat crew, finally filling a position that was open for quite some time, and brings forth an even stranger character. Triumph and forboding mark the ending of this arc as the crew celebrates their victory but at the same time comes to realize the dangers that await them in the Grand Line.
This arc is not only a fan favorite of One Piece, but it also holds a place within the bigger scheme of the One Piece narrative. It introduces Brook, an essential part of the Straw Hat crew; world-expansion with the Seven Warlords of the Sea; development in terms of strength and teamwork—specifically, with the battle against Oars; and really sets up some future plot and character development, especially in Zoro's case, where the readers realize just how much he's willing to sacrifice for his captain and crew.