Chapter 664 is titled "Master Caesar Clown".
While Smoker, Tashigi, and the G-5 Marines try to determine the reason for the children's presence on the island, the Straw Hats examine Brownbeard and learn more about the history of Punk Hazard. Caesar Clown then uses one of the defeated centaurs as a test subject.
Tashigi recalls hearing about reports of child abduction in G-5's jurisdiction. The soldiers are finding it difficult to get used to their superior officers' new appearances. Smoker tells her he does not know about any kidnappings. Tashigi begs him to button her shirt so her chest would not be exposed. Tashigi recalls getting reports of a lot of accidents, and a soldier remembers that a lot of kids die on the high seas. Tashigi remembers seeing a lot of reports about kidnappings from the G-5 communications department. By the time the stories reached the newspapers, they had been changed to accidents at sea. Tashigi starts talking about what it could possibly mean, apparently stumbling around her point. Smoker gets irritated and tells her to just say what she is thinking. Given the number of children they just saw, the kidnapping reports would be true. Smoker then realizes that the kidnapper would be on the island. If true, then the articles printed in the newspapers were fraudulent. That would further mean that someone from inside G-5 is trying to cover up the stories. The soldiers are furious at the thought of there being a traitor among them. Smoker tells them to shut up, saying that none of them are worthy of being suspected. Smoker tell them that by thinking that a perfect society exists, they overlook the enemy. Smoker tells them that since Law has his heart and is not in his own body, he cannot leave the island. He orders that a third of the soldiers steal a ship and return to G-5. Smoker tells them he will find his heart, get back in his normal body, and expose the mastermind behind the whole scheme.
In the research lab, Caesar puts down a transceiver and tells the guards present that they have permission to kill the Straw Hats, the G-5 Marines, and Kin'emon, the samurai, adding that if someone dies on the island Joker will make it look like an accident at sea. He orders the guards to capture anyone they can, and he does not mind if the ones captured get poisoned. He asks where Law is, and is told that he just left. He then reminds the guards that their enemy is worth 400,000,000, so it will not be easy. Monet interjects, saying that the other bounties range from 50-120,000,000, totaling 800,000,050. The crew suddenly became inactive two years ago, meaning the current figures might not be good indications of their threat level. Caesar comments on the harpy's diligence. One of the centaurs stumbles in from the cold. He tells Caesar that the Straw Hats took some of their coats. He also tells them they took Brownbeard, and begs Caesar to get him back. Caesar tells him not to worry about their boss, and that in the back room, there is a new medicine they just completed that will warm the body and heal wounds. The centaur is grateful, and asks that his friends be treated too.
The scene changes to the Straw Hats. Usopp has made special buttons and placed them on the foreheads of the four people not in their correct bodies. On the buttons are pictures which indicate who is in which body, making them easier to identify. Luffy is talking to Brownbeard, saying he has never heard of him. Zoro comments on how a moment ago, Brownbeard was set on keeping quiet. Brownbeard adds that Law will come save him soon. He adds that Law is different than pirates from his and Luffy's generation. Luffy does not understand, and Brownbeard explains that the "Worst Generation" refers to the eleven rookies that gathered at Sabaody, as well as Blackbeard. They are problem-children born between two eras who carry the fate of the war. After the war, these pirates invaded the New World and wreaked havoc. Any major incident involved pirates from that group. Brownbeard then goes on to say that the one who shattered his crew was Basil Hawkins. Brownbeard lost the use of his legs from that fight, so he and his crew fled, eventually arriving on Punk Hazard. He explains that Punk Hazard used to be full of lush green vegetation, teeming with life. He goes on to say that it used to be Vegapunk's experiment facility where he tested weapons and drugs. Some convicts were brought to the island, instead of prison, to be used as guinea pigs for his human experiments. Then, four years ago, one of Vegapunk's chemical weapons experiments failed, demolishing two of the three laboratories. The explosion brought about extreme heat and sent poisonous particles into the air, killing everything. The government workers abandoned the prisoner and sealed off the island before making their escape. The prisoners flocked to the only research building still standing. If they did not die, their lower bodies became useless due to a powerful nerve gas. All seemed lost. A year later, Caesar Clown came to the island. He used his ability to purify the gas, and made artificial legs for those who could not walk, whom he also made his subordinates. Brownbeard arrived on the island a year after Caesar did. There was still poisonous gas, which made him incredibly sick. Just before giving up on life, Caesar and his subordinates appeared and saved him.
Brownbeard compares Vegapunk to the Devil and Caesar to God. A few months before the Straw Hats arrived, Trafalgar Law did. Using his ability, Law gave all the disabled by the gas, living animal legs, allowing them to walk again. Nami comments on how cruel Vegapunk was while the rest of the crew applies the story to what they had seen so far, the satyrs from the lab, as well as the harpy and other centaurs. Usopp asks him about the dragon. Brownbeard tells them that the dragon was an artificial creature made by Vegapunk to guard the island. It can adapt to any environment. One of the Celestial Dragons named it at one point, but Brownbeard cannot remember what it was. He tells the crew to run if they meet it, Zoro just tells him they will, despite remembering having killed it. Brownbeard goes on to say that the island belongs to Caesar Clown and no one dares to come near it. Caesar still continues his research for the good of mankind even today, and the Straw Hats will have to submit their bodies to his experiments every now and then. He then laughs and says they (which includes himself) will not let the Straw Hats escape.
Back inside the research facility, the centaur is having trouble breathing. He is banging on a door, asking Caesar what kind of medicine he was given, begging to be let out. Caesar tells him to be a patient. He asks how the centaur is feeling. Based on the sounds he made, Caesar deduces the centaur is nauseous and coughing. Caesar asks if he has a headache, and is pleased to hear he does. He then asks if the centaur is hallucinating. The centaur tells him there is a giant snake in the room, but Caesar assures him there is not. He then comments that the experiment was not going as well as he hoped. Monet asks him if it was another failure. Caesar snaps at her to watch what she says, as his experiments do not fail. And he applies the same statement to the incident four years ago. He asks if creating a weapon that could kill an entire island is really a failed experiment. He laughs, saying he even kept all the victims under his gaze. He then tells the World Government to watch, and he will show them who the real number one scientist is.
The scene changes to the Marines, where a soldier is stunned by what Smoker just told him. The one responsible for the incident on the island was in fact Caesar Clown. Caesar made the weapon and was responsible for letting it go off accidentally. Smoker explains how he and Vegapunk were always rivaling each other. However, developing a weapon for mass murder that spreads poison gas is illegal. Caesar was caught and arrested. However, he escaped the prison ship and was never seen or heard from again. Smoker then gives the postulate that if Caesar is indeed furthering his research on the island, if he is not stopped now, then another incident like the one before will not be far away.
- Smoker and Tashigi suspect that someone inside G-5 is falsifying the kidnapping reports happening in the area.
- The name of the samurai from Wano Country is revealed to be Kin'emon.
- Someone named "Joker" gives Caesar Clown permission to kill the Straw Hat Pirates, Kin'emon, and the G-5 Marines. Caesar says Joker will make it look like an accident at sea.
- Usopp makes special markers for Nami, Sanji, Chopper and Franky to more easily discern which person is in what body.
- The outcome of Brownbeard and Basil Hawkins' battle two years ago is revealed. Brownbeard was defeated and he lost the use of his legs.
- Brownbeard explains to the Straw Hats about how Punk Hazard became a wasteland.
- Brownbeard also reveals that the dragon encountered by Luffy's group was an artificial being and that one of the World Nobles took a liking to it and gave it a name, Brownbeard forgot the name though.
- The failed experiment that ravaged Punk Hazard is revealed to have been created by Caesar Clown and not Dr. Vegapunk, and that Caesar intentionally set it off.
- According to Brownbeard, the centaurs, and satyrs were originally humans who were altered by Law's ability after losing the use of their legs.
- Caesar Clown's face is finally shown. He is a Logia Devil Fruit user who ate the Gasu Gasu no Mi and has a bounty of 300,000,000.
- first introduction