A girl named Myskina Olga is pursued by both the World Government and a man named Mad Treasure, as she is the only one who knows the location of the Pure Gold, a substance that can buy the entire world. Olga ends up sailing with the Straw Hat Pirates, and they journey to find the Pure Gold on the lost island of Alchemi, which is located inside the stomach of a large anglerfish named Bonbori. Olga and Luffy get separated from the others, and Olga reveals that she has lived for 200 years, and has a grudge against her father for making the Pure Gold, which led to the death of their mother and them being eaten by Bonbori.
Mad Treasure and his crew enter Bonbori, and capture Olga, Nami, Usopp, Robin, and Chopper. Meanwhile, Luffy forms an alliance with Olga's father Myskina Acier, and they, Zoro, and Sanji go to rescue their captured allies. Mad Treasure takes his prisoners into a mine where the Pure Gold is located, but Luffy and Acier intervene as the Straw Hats and Olga find the Pure Gold. Olga then discovers that Acier created the Pure Gold to prevent the onset of an illness and save her life, and the pirates are forced outside as Acier destroys the mine. Mad Treasure gets his hands on the Pure Gold, and Luffy battles him as Olga and Acier reconcile. Luffy beats Mad Treasure, and Zoro and Sanji beat Treasure's subordinates Naomi Drunk and Psycho P before they escape from Bonbori. The Pure Gold is fully absorbed by Bonbori, but everyone is fine as Olga and Acier resume their normal lives and the Straw Hats get a Vivre Card that leads to Gran Tesoro.