The Millenium blue arc is a story arc.
The arc begins when the Straw Hat Pirates enter the Millenium Blue, having a peacefull day, until a World Government assassin suddenly appears on their ship. Altough he appears to have devil fruit powers, Luffy is able to knock him into the water before he may hurt anyone. Nami then tells them to turn the ship around, but behind them a giant seastone wall has appeared - escape becomes impossible. Everywhere around them Den den Mushi are activated and cry out: "Here is the holy World Government! A pirate ship appears to have entered Millenium Blue! The hunting season starts now!" While Straw Hats are shocked, a terrified Nami tells them what she has heard about the Millenium Blue: it is a cruel place owned by the World Government and the Marines. Whenever a pirate ship enters the Millenium Blue, its fate is sealed and its crew will be captured within a week. Meanwhile, a pirate captain is sitting in what appears to be an undersea cave. An envoy from someone tells him that his allies, four of the Five Predators, will go to greet the newcomers, but instead of following his allies, he crushes the envoys head with a giant nunchuck. His subordinates then throw the half-dead envoy into a lake and pay their respects to their great captain, Jiguro. On the Thousand Sunny, Usopp spots three pirate ships coming closer. Their captains are Eneas D. Fatigo of the Odyssey Pirates, Xiang D. Yu of the Chuhan Pirates and Joel Hallyday. Franky, who has heard about them, is shocked and refers to those three as "legends". He then tells his fellow Straw Hats that those captains belong to the few who once fought Gol D. Roger without getting killed. On his ship, Eneas D Fatigo is watching the Thousand Sunny and states that he will try not to destroy such an awesome ship, but that each member of its crew will die if he does not surrender to him. While the three ships start to circle the Thousand Sunny and their crews start to fight the Straw Hats, a fourth ship appears on the horizon, its leader being the "lonesome pirate", Lord Drav. Drav states that he was too busy to arrive earlier, but is suddenly pierced through by a ray of light coming from behind him. On the Thousand Sunny, Roronoa Zoro has to fight Biu Lang, but is able to knock him out, while Brook and Usopp encounter Joel Hallyday, who displays his Jelly Jelly powers and corners them. Luffy jumps onto the Odyssey Pirates ship and defeats many of them, before their captain, Eneas, attacks him with his giant spear. Nami and Franky fight the Odyssey Pirate Marchosias, who uses his puppet soldiers and needle skills, but is then defeated by Franky with a Coup de Vent. Sanji, who tried to help them, fights Xiang D Yu, being knocked down again and again. Chopper and Robin have to reason with Zoo, who appears to use judo. Suddenly, a huge explosion starts on the Odyssey Pirates ship, and Luffy nearly falls into the water, while eneas floats out of its range. Marine Admiral Kizaru enters the battle, using his Pika Pika no Mi powers to kill one pirate after the other, beginning with Odyssey Pirates cabin boy Barbatos, who isn´t even able to touch him. Altough Luffy is shocked by the Admirals presence, Eneas states that in Millenium Blue, one should always expect such things, not at least because Kizaru is one of its rulers. Meanwhile, a strange squad of fighters has arrived behind Kizaru, who states that they are the Millenium Blues Special Deletion Unit. Looking one last time at Luffy, Eneas tells him to go back on his ship and to hand over the defeated Odyssey and Chuhan pirates. Then, Eneas and Xiang D Yu jump towards Kizaru, being followed by their crewmembers. Joel Hallyday, who is still on the Thousand Sunny, then laughs about their mindless effort, revealing that he is a Marine spy and a member of the Special Deletion unit. Encounering all of the Straw Hats, he decides to aid Kizaru in defeating Eneas and Xiang instead of "messing around with midgets". Altough Luffy wants to fight Kizaru, the Straw Hats escape. On their ship, Sanji states that, before facing Kizaru, Xiang D Yu told him about the Exclusion Key, an instrument that could allow the Straw Hats to leave Millenium Blue, and that it was located in Arbalessia, the great marine base in the middle of Millenium Blue. The Strawhats then decide to get the mysterious key and to flee from this hell.
The Millenium Blue arc will be continued in the Arbalessia Fortress arc