The crew chases after Nami, who stole their ship, and Yosaku takes time during the trip to explain about Arlong and his Fishmen to Luffy and Sanji. Meanwhile, Usopp, Johnny, and Zoro arrive at the island Nami docked at.

Zoro is captured by the fishmen after Usopp and Johnny ditch him, and it's revealed that Nami was part of Arlong's crew the entire time. When a group of Fishmen attack Cocoyashi Village, Usopp comes to the rescue.

After Zoro escapes with Nami's help, she tries to prove her loyalty to the Arlong Pirates by appearing to kill Usopp. Meanwhile, Luffy's group finally arrives at the island.

Johnny tells the others about Nami killing Usopp, and Luffy does not believe them until Nami appears herself and confirms it. She tells them to stay out of her business and leave without her, but Luffy stubbornly decides to stay on the island. Usopp returns, revealing that Nami only pretended to kill him, and Nami's sister, Nojiko, starts to explain why Nami is acting the way she is.

Nojiko starts her story about Nami's childhood and their adopted mother, Bellemere. Despite being poor and none of them being related by blood, they had a happy life.

Nojiko continues her story, explaining how their life was ruined when the Arlong Pirates first took over the village.

When a marine officer under Arlong's pay confiscates all the money Nami stole over the years, in order to one day buy the village from Arlong, the villagers decide that enough is enough and fight back, despite Nami's pleads that they do not and that they'd all be killed. Upon seeing Nami's sorrow, the Straw Hats head for Arlong Park.

The Straw Hats face the Fishmen themselves as the villagers watch from the sidelines.

Arlong has trapped Luffy in the ocean and Zoro faces the six-sword wielding octopus Hatchan.

With Hatchan defeated, two of the fishmen battle Sanji and Usopp. Things look grim for Sanji as he attempts to face Kuroobi underwater, and Usopp gets the chance to prove that he too is a brave warrior.

Arlong's officers have been defeated, but Sanji and Zoro prove to be no match for Arlong in their current state. However, with Luffy finally free, he just may be able to defeat the "most wicked man in the East Blue". Meanwhile, Nami finally gathers the courage to face Arlong.

Luffy returns to the fight, and the battle with Arlong versus Luffy begins.

The battle spreads into Arlong Park itself and Arlong is no longer holding back.

Arlong's reign of terror is over, and no one has anything on their mind except to party. As the crew celebrates, Nami reflects on her hometown and decides to officially join the Straw Hat Pirates.

As a result of defeating Buggy, Krieg and Arlong, Luffy now has the highest bounty in all of East Blue. News quickly spreads to Shanks, who celebrates Luffy's arrival into the world of pirates by partying with Mihawk, despite already being severely hungover.