Chapter 998 is titled "Ancient Types".
Marco uses his phoenix flames to halt the spread of Ice Oni, giving Chopper more freedom to produce the antibodies. Scratchmen Apoo attempts to take back the virus antidote, but is attacked by X Drake in his allosaurus form. With enough allies to protect Chopper, Zoro, Robin, and Brook decide to have Marco fly them up to the dome's roof.
Elsewhere, Sasaki prevents Franky from attacking his subordinates by transforming into a triceratops and charging into him. On the third floor, Black Maria has transformed into a giant prehistoric spider and pursues Sanji as he tries to get away, wanting to make him her pet. On the fourth floor, Jinbe stays to fight the horde of Beasts Pirates there while Luffy goes on ahead, and after Luffy leaves Jinbe confronts Who's-Who. Outside the dome, Page One and Ulti resume hunting down Nami and Usopp after defeating Hihimaru.
Meanwhile, Yamato and Momonosuke have hidden out in a storeroom, where a big dragon statue is located. Yamato reveals to Momonosuke that the statue was damaged by Ace when he came here to kill Kaido a few years ago.
The Beasts Pirates are left stunned to see Onigashima being levitated into the sky by Kaido before turning their attention to Marco's arrival. Marco uses his phoenix flames on Chopper which concerns Zoro and Brook, but after Marco dodges them, Chopper realizes that even though he is on fire, he isn't feeling any pain, and that the fire is at the right temperature to hold the virus back. Miyagi and Tristan arrive telling Chopper they are ready to produce more antidotes for Ice Oni, which pleases Chopper. Marco sees the other Ice Oni and uses an attack called "Nashi no Tsubute" to melt the ice on the infected, even Omasa regains control of himself. Marco tells Chopper to work fast because if they run out of stamina, they will turn back. Queen, while watching, becomes angry at Marco for his interference and tells his men to steal back the antidote, claiming the enemy won't bother trying to save them. Chopper proceeds to start making a cure.
After gaining an understanding of the situation, Marco offers to fly Zoro, Robin, and Brook up to the top of the roof. Brook is concerned about Chopper but their allies offer to keep him safe. Scratchmen Apoo gets back up and tries to reclaim the antibodies from Chopper. X Drake, however, stops Apoo by grabbing him with his mouth in his full Allosaurus form and throwing him to the ground. Drake tells Zoro that if he sees him as an ally, then they can go and leave the floor to him. Zoro accepts this.
On the fourth floor, Jinbe says to Luffy that the reason Sanji stayed on the third floor is most likely because he sensed a very powerful enemy, causing Luffy to remark that he needs to further train his Kenbunshoku Haki. Jinbe also sees that the fourth floor is filled with tough enemies and offers to stay behind and let Luffy continue on. At the right-brain tower, Franky fires at Sasaki's troops with General Franky who block his missiles before Sasaki charges at Franky and knocks him over. After he recovers, Franky sees that Sasaki has transformed into a Triceratops, having eaten the Ryu Ryu no Mi, Model: Triceratops. Sasaki and his troops tell Franky to give up because he is the strongest animal in existence.
On the third floor's banquet hall, Sanji, while still tied up, is running from Black Maria and her group, with Black Maria revealing she has eaten the Kumo Kumo no Mi, Model: Rosamygale Grauvogeli. She teasingly tells him to stay forever so she can make him her pet. Sanji tries to keep his cool but in his eyes, he is completely surrounded by girls, which he considers heaven. This causes all the women though, to say that something is wrong with Sanji. Jinbe enters the Nekoka-fé on the fourth floor and confronts Who's-Who, who has eaten the Neko Neko no Mi, Model: Saber Tiger. He seems to be a little disappointed that Luffy has gone on to bother Kaido and demands Jinbe leave his subordinates alone while calling him a former Warlord of the Sea. Jinbe tells Who's-Who that he would prefer to be referred to as the Straw Hat Pirate's helmsman now, causing Who's-Who to sarcastically apologize and that he called him that because he once saw him when he was a Warlord. Jinbe tells Who's-Who that if he takes his mask off, he might remember him, but Who's-Who tells him that will never happen. Outside the Dome, Page One and Ulti have defeated Hihimaru and are currently fighting the samurai while wondering where Usopp and Nami went, with Ulti wanting to smash Nami's head.
Yamato brings Momonosuke and Shinobu to the Skull Dome's storage room to treat her, confident no one will find them there. Momonosuke notices a statue and asks Yamato what it is. Yamato explains that it was a statue of a dragon that was previously placed at the entrance of Onigashima, but it was discarded after their friend destroyed it. That friend turns out to be Portgas D. Ace, who Yamato reveals came to Wano many years ago to kill Kaido.
- Marco uses his flames to suppress the effects of the Ice Oni.
- Omasa regains control of himself but is still covered in frost.
- Chopper begins mass-producing the Ice Oni antidote with Tristan and Miyagi’s assistance.
- Apoo recovers and attempts to retrieve the antidote but is stopped by Drake.
- Jinbe decides to remain on the fourth level to hold back the enemy while Luffy goes forward.
- It is revealed that the remaining Tobiroppo have all eaten Ancient Zoan fruits.
- Sasaki ate the Ryu Ryu no Mi, Model: Triceratops, a fruit that allows him to transform into a full Triceratops.
- Black Maria ate the Kumo Kumo no Mi, Model: Rosamygale Grauvogeli, a fruit that allows her to transform the lower half of her body into a Rosamygale Grauvogeli.
- Who's-Who ate the Neko Neko no Mi, Model: Saber Tiger, a fruit that allows him to transform into full a Saber-toothed Tiger.
- Jinbe confronts Who's-Who, who once saw Jinbe as a member of the Seven Warlords of the Sea.
- Hihimaru is revealed to be defeated, and Page One and Ulti resume pursuing Usopp and Nami.
- Yamato reveals that Ace once came to Onigashima to kill Kaido.