As an abridged version of the Arlong Arc, the special follows the same general plot. Nami leaves the crew after the Baratie Arc to go back to her village. Luffy and the crew follow her, and discover that she is working for a dangerous Fishman pirate named Arlong. It turns out that Arlong killed Nami's mother and has been using her genius cartography and navigating skills for the past 8 years for his own benefit; and the only way out is to pay him Beli100,000,000, which she gets by stealing from pirates. When Luffy finds out, he and the crew defeat Arlong, proving that Nami belongs with them, and they set sail to explore the rest of the sea.
At the end of the special, it is revealed that the retelling was actually Nami herself reminiscing post-timeskip while the crew was heading for Fishman Island.