Chapter 531 is titled "Level 3 Starvation Hell".
The chapter opens with the Marines going into an absolute frenzy, since the high security prison Impel Down has been successfully invaded by a lone pirate. The make it their number one priority to capture the culprit. This where we are formally introduced to the chief of Impel Down "Sadi". She mentions that the lower half Impel Down to be a hellish labyrinth that a man with no real idea of where the were going will only end being collateral damage. Before ordering to simply fortify the surroundings, effectively sealing out the outside world. Behind Sadi-chan was a Koala apparently sent by her to hunt down Luffy.
Meanwhile, Hannyabal and Magellan escort Boa Hancock to where Ace is, before introducing her thus, "Both powerful and sublime! The world's most beautiful woman!! The Pirate Empress Boa Hancock herself! Whoo! Whoo!" Jinbe and Ace seem shocked while the other prisoners there with him begin to riot, excited simply by her presence and beginning to speak excitedly about Amazon Lily. Ace starts with the question "What do you want with me?" while Jinbe is disgusted with her for lending a hand to the World Government. All the while the prisoners continue to riot, which she aggravates further by giving a cute face and saying "I shall be afraid." Magellan then puts it down by using his poison Hydra, urging to finish her business quickly with Ace. She then quickly turns around and says that she is already finished with him. Ace is shocked and asks her is she is serious, but Hancock simply replies with, "I see no reason why I should lie". Magellan and Hannyabal stand around puzzled by the conversation wondering if the other heard it. Jinbe also asks him this question, to which Ace replies with a deeply troubled face, "My brother..! She said he was here." Jinbe is also shocked and mentions that is deeply reckless of him to do.
In the Starvation Hell, Luffy wonders around deeply lost to where he is, while the Marines suddenly attack. Luffy gets in trouble with the lion creature before seeing Zoro, much to his delight. Zoro, meanwhile says, "Not quite." It turns out to be Mr. 2 Bon Kurei, much to Luffy's further delight and disappointment. Together they defeat the creature and celebrate in meeting each other again. Where he mentions that he escaped death my stealing an appearance of someone else, and tells him he would like to meet someone in Level 5 and that they should go together.
- Sadi and a koala are introduced.
- Hancock passes on a message to Ace, informing him that Luffy snuck into Impel Down to try to rescue him.
- Luffy continues wreaking havoc through Impel Down.
- Luffy gains another ally in Impel Down, Bon Kurei.
- Bon Kurei says he wants to meet someone on Level 5.