Cover Page: "Franky, Riding on a Mecha-horse he Built, has a race with a (wild) Mustang", P.N. Magic Mushroom-san.

The Straw Hat Pirates and the pirates on the Flying Dutchman are escaping from the erupting volcano. As the other pirates, pulled by Wadatsumi and Ankoro, flee out of sight, the Straw Hats jump into a dark trench with Surume leading. They now fall, with an avalanche of stones following them that are getting closer to the Thousand Sunny. Right before being hit, Usopp saves the crew by using a Midori Boshi: Sargasso, growing seaweed on the cliff, catching most of the rocks. However, some rocks fall on the head of the Kraken, and they plunge into the deep sea, 10 000 meters below. Sanji, thinking of mermaids, almost faints. Being surrounded by enormous sea creatures, three fish-men come close. Luffy is asked to become a subordinate to the New Fish-Man Pirates, while Usopp fuels the tank to do Coup de Burst to Fish-Man Island. The chapter ends with Luffy turning down the offer.

The crews of both the Thousand Sunny and the Flying Dutchman realize that a volcano is about to erupt. Vander Decken tells his crew that they are retreating for now. He then commands Wadatsumi and Ankoro to pull the ship to safety. Nami realizes the full danger of the situation and tells Luffy to tell Surume to get the ship as far away from the volcano as possible. Luffy starts to call out the orders, but Usopp says he does not have to since Surume is already running away. Luffy says he wants to see the volcano erupt. Usopp tells Surume to escape to the front side, but Nami says it would not matter since it is only a few meters of difference. The volcano erupts. As the magma is making its way down to the ocean floor, Nami realizes that the water temperature is rising. Everyone looks up in time to see Wadatsumi and the Flying Dutchman get thrown around as the temperature differences caused by the magma create whirlpools. Sanji asks Nami which way they should go to get to Fish-Man Island, suffering a nosebleed immediately after asking the question. Chopper is surprised that he would get a guilty conscience in such a dire situation. Nami replies that they have to move forward a little more in order to get into a specific trench.

The trench in question was a precipice preceding a large drop off into total darkness. The crew could not believe that Fish-Man Island was really at the bottom of the trench. With the water heated by the lava catching up to the ship, Usopp exclaims that the darkness is scary and that he does not think will leave the trench alive. He tries to tell Luffy that the darkness will take them to Hell, but Luffy yells to Surume to jump in. The kraken jumps in just as the volcano lets out another burst of magma. Franky then notices something above the ship. It is a rockslide caused by the volcanic activity. Nami exclaims that the volcano has just destroyed the trench. A rock is falling towards the ship. Zoro orders Surume to get out of the way, as he is going to cut it. Before he can leave, Robin grabs him using her Devil Fruit ability, saying at 8,000 meters below the surface, the water pressure would crush him. Usopp then unveils a new attack: Midori Boshi: Sargasso. Seaweed sprouts from the rock face, catching the falling rocks. As Chopper looks in awe, Usopp says his attack will stop the rocks momentarily, leaving enough time for Surume to get away. Surume gets out from under the caught rocks just in time, as the plant holding them gives way. While everyone is praising Usopp, they do not notice the errant boulder falling toward Surume. The boulder hits Surume above the eyes, stunning him, possibly knocking him unconscious.

An narration box gives the crew's location as 10,000 meters below the surface, calling the bottom of the trench the ocean depths. In the same panel, Surume is lying in a heap and light is streaming down from above. Thousand Sunny is on its side but still on top of Surume's head. Everyone is grateful to be alive. Robin asks how there could be light so deep in the sea. Brook makes a joke saying the light is blinding him but that he does not have eyes. Franky calls everyone over to one side of the ship and tells them to look up. Usopp is having trouble seeing what it is since the light is so bright. Luffy asks Nami if that is it. Nami replies without a doubt, as the needle is pointing directly at it, that they have reached Fish-Man Island. They note that the island is surrounded by a large bubble, indicating that there is air on the island. Everyone cheers. Caribou, from his barrel imprisonment, also notes their arrival while laughing. Sanji states that he has wanted to visit this place since he was a kid, the place where beautiful mermaids dance. As he starts cheering, he succumbs to another nosebleed. Usopp and Chopper realize that the rehabilitation did not help Sanji in time, as all he had to do was fantasize about mermaids in order to get a nosebleed. Chopper suggests that it is not the best idea to meet a real mermaid now. Usopp argues saying that Sanji's dream is to meet a mermaid. Chopper replies that meeting one will cost him his life. Sanji says it does not matter, much to Chopper's surprise. Sanji goes on to say that he would rather die while looking perversely at mermaids than live a life where his dreams did not come true. Sanji declares that he will meet the mermaids. Luffy, meanwhile, is wondering what kind of meat goes into the food on Fish-Man Island. Nami tells him to wipe the drool from his chin and help them find the entrance. Franky then asks what some off-screen figures are doing there.

A voice asks Surume why he is letting humans order him around. Surume freezes in fear, then tosses Thousand Sunny up into the air and starts to run away. The ship is then blocked by six sea monsters resembling a rhino, a giraffe, a gorilla, an elephant, a lion, and a bear. Nami, Usopp, Chopper, and Brook look at the massive creatures and exclaim that it is all over, despite making it this far. A figure riding on the giraffe-like sea monster says that since they are pirates, that means they are enemies of the humans, which means he is about to give them a choice. He then recognizes the Straw Hat crew by their jolly roger. The person speaking said he had heard a lot about them and recounted their defeating the Arlong Pirates, about how they saved and protected Hatchan, and about Luffy punching a Celestial Dragon, and then compared Luffy to Fisher Tiger. The person talking is revealed to be a tall, pike eel fish-man named Hammond. He is a combatant under the New Fish-Man Pirates. He was accompanied by a merman and a fish-man. Hammond asks the crew if they are friend or foe, if they fall under the control of the New Fish-Man Pirates or reject them. Hammond adds that if they refuse, they will be sunk then and there. Hammond has the jolly roger of the Fish-Man Pirates (Arlong's old crew) tattooed on his neck by his left shoulder. The sight of it seems to make Nami uncomfortable, and she tells Franky to refuel the ship. Franky asks what the fish-men think they are doing. Nami says that at 10,000 meters below sea level they have absolutely zero chance of winning in a fight against them. She tells Franky her plan. They are going to run away by using the air they have left in the ship to pull off one last Coup de Burst and charge right into Fish-Man Island. Much to Franky's disbelief, Nami said it was either that or die then and there. Robin agrees that that is their only option. As Franky handled the steering, Usopp refilled the fuel. Just as the Coup de Burst was at full power, Hammond asks Luffy one more time if he would become their subordinate, but Luffy refuses.

Chapter Notes:
Usopp reveals another Pop Green which sprouts giant seaweed.
Usopp also appears to have upgraded his Kabuto weapon to a new form.
Sanji loses more blood thinking of mermaids.
Fish-Man Island is seen for the very first time.
The New Fish-Man Pirates are introduced, and a figure of some kind of authority in their ranks, Hammond, is named.
For the first time, a merman is shown in the series.
Surume is revealed to have already been tamed, however Luffy is the first human to tame him.
Hammond has a tattoo of the Arlong Pirates' jolly roger and another tattoo, an unknown symbol with a decapitated human that resembles the jolly roger of the Sun Pirates, which is later revealed to be the symbol of the New Fish-Man Pirates.
Luffy rejects the New Fish-Man Pirates.
The chapter title is possibly a reference to the novel 20,000 Leagues Under the Sea.
Robin says Zoro's name (in the manga) for the first time, meaning the only crew members she has yet to address by name are Sanji and Brook.