The G-8 Arc is the fifth filler arc in the anime One Piece. It comes after the end of the Sky Island Saga. Instead of splashing down in the open sea, the Going Merry is trapped within the enclosed and heavily fortified waters of a marine base, where the crew must scatter to survive before attempting to retake their ship and escape. This filler arc is currently the longest filler arc, containing 11 episodes.
When the Straw Hats land in the middle of the marine base G-8, they are forced to abandon their ship, as many marines immediately spring upon it. Separated, the crewmates are forced to disguise themselves and try to find a way out of the base, which is comprised of a huge mountain surrounded by water, and then a ring of sheer rock cliffs that form a wall of sorts.
Sanji and Luffy get by whilst impersonating a pair of famous cooks named the Marley Brothers, who arrived at G-8 around the same time as the Straw Hats. There they meet Jessica, the head cook, whom Sanji promptly falls in love with and begins to fawn over. When she insults his cooking, however, he decides to show her the true meaning of "cooking from the heart". He impresses all of the cooking Marines with his skills and shows Jessica that a good chef avoids wasting food.
Meanwhile, Nami escapes on her Waver and disguises herself as a janitor. In this disguise, she manages to give the real Marley Brothers improper directions to the kitchen, causing them to become lost in the huge, mazelike fortress. She also accidentally gets Zoro captured by the Vice Admiral of G-8, a laid-back man named Jonathan.
Usopp and Robin wind up in the same place, and while Usopp wants to quickly run away and find the Going Merry, Robin decides that they would be better off gathering information about G-8. Usopp refuses, and the two go their separate ways. At the same time, a Special Investigator named Shepherd has arrived from Marine Headquarters with plans to reveal G-8's uselessness. G-8 is considered a "peace crazy" naval base because it is so massive, yet rarely has to deal with any trouble. Before he can meet with Commander Jonathan, however, he is attacked by Robin and left unconscious in a hallway.
Usopp steals some clothes and disguises himself as a marine rookie. He finds the Going Merry but is stalled by a shipwright named Mekao, who chats with Usopp about how the Going Merry needs serious repairs or it will be unable to sail. Suddenly, Lieutenant Drake appears, one of Jonathan's subordinates. He tricks Usopp into admitting he's a pirate. Usopp tries to lie his way out of the predicament several times, but ends up blundering each attempt. When Robin arrives in the Vice Admiral's office disguised as Shepherd, Usopp pretends to be her assistant. She says he's lying and Usopp is thrown in the brig with Zoro.
Tony Tony Chopper is discovered by some marines who think he is a "good luck tanuki" and pursue him. He manages to hide in a box full of medical supplies, which is then brought to the medical center. All of the doctors have been called away from G-8 except for one, a pediatrician named Dr. Kobato who cannot stand the sight of blood. Unfortunately, a ship full of injured Marines has docked at G-8 and she has to operate on them. Seeing her dilemma, Chopper disguises himself and helps her. Nami, who is now disguised as a nurse, reunites with Chopper and also assists Kobato.
Because of Sanji's foolishness, he and Luffy are still working in the kitchen. When Jessica asks Luffy to deliver a plate of food to her husband, neither of them thinks anything of it, until they learn that Jessica's husband is none other than Commander Jonathan! He sees Luffy and recognizes him as the 100,000,000 bounty. Sanji and Luffy are surrounded by cooks and must flee, their cover blown.
The real Agent Shepherd is found unconscious in the hallway and assumed to be a Straw Hat. When he's dragged down to the dungeons and thrown in jail with Usopp and Zoro, Usopp lies and says that Shepard is their crewmate, "Condoriano". Shepard continues to claim he's really a Naval officer, so Zoro KOs him with his forehead.
Luffy and Sanji at this point have learned Zoro is in jail and go to free him. However, Jonathan has predicted this and has Lieutenant Drake set up an ambush. When the two arrive, they are surrounded, and Luffy cannot break the jail's bars because they are made of Seastone. Usopp and Sanji work together and blow up the prison cell, escaping with Zoro and Luffy in tow. They try to cross a huge bridge connecting the mountain to the cliffs, but are attacked by a group of marines called the "Hot Wind Marines". Luffy breaks part of the bridge and everyone falls into the ocean.
At this point, everyone except Robin is heading toward Dock 88, the Going Merry's location. When Luffy, Usopp, Zoro, and Sanji arrive, they discover that Jonathan has predicted this move as well, and sent Drake ahead. They are vastly outnumbered, and things get worse when Luffy is trapped into a net made of Seastone. Luckily, Chopper and Nami arrive at that moment with Dr. Kobato, and use her as a hostage to get onboard the Going Merry. Makao sees this all play out and decides to release the Going Merry into the water. It turns out that Makao is Kobato's father. One of Drake's men grabs Kobato but since Nami is dressed like a nurse, the marines think Luffy and crew still have a hostage, and let them go.
Robin shows up with a map of the base and the Straw Hats hide Going Merry in an abandoned dock, since there are so many. Usopp uses junk in the dock to disguise Merry as a Naval ship, and then splits the crew into two teams: one team to recover the treasure gained at Skypiea, and the other team to retrieve Nami's Waver. Chopper is left behind to guard the Merry.
Jonathan tricks the crew into another ambush, this one in a huge room that is locked from the outside. Drake is once again leading the attack, with countless marines backing him up, and things look bleak until Agent Shepherd foolishly opens the doors and allows Luffy, Robin, Zoro, and Usopp to escape.
Nami and Sanji get the Waver and go back to Merry; Nami orders Sanji and Chopper to bring the ship out into the waters and wait for the others to return. She goes looking for Luffy and finds him, only to discover that they still do not have the gold. Nami suspects that it's in the Commander's office, so she and Luffy go alone while everyone else goes back to the boat. Using the Waver, she and Luffy scale the main mountain and break into Jonathan's office. Marines fill the room but Nami uses her Thunder Tempo attack to electrocute them all. She and Luffy grab the treasure and head back to the boat.
However, Jonathan is not worried, because at 9 PM the "fangs of G-8" will appear. What he means is the inner lake becomes so shallow that the Going Merry is stranded on a reef. With no water, the Merry is stuck in the mud while the marines mobilize.
Usopp produces an Impact Dial and Luffy produces the same octopus that the Straw Hats used to exit Skypiea in the first place. Along with a Breath Dial and a Flame Dial, they manage to blast the Going Merry into the sky and use the octopus as a balloon once more, sailing over the cliffs of G-8 and escaping, once and for all. While Commander Jonathan is disappointed, he explains to his wife that the pirates brought G-8 and its inhabitants together, and gave them some much needed exercise.