Chapter 604 is titled "Into the Depths".
As the Straw Hats venture deeper into the ocean, Nami, with some help from Franky and Brook, explains how different parts of the underwater current system work. Caribou and his crew try to attack the ship, but due to Momoo, becomes the only one of his crew to make it onto the ship. He pleads for his life and the crew ties him up. The chapter ends with the crew sighting the gigantic deep sea current, confronting a kraken, and Luffy planning to make the massive creature his pet.
The chapter starts with the crew wondering how far the ship had descended below the ocean. Usopp comments that the light almost cannot reach them anymore. Franky said they are past a level called the Euphotic Zone and are about to be out of another level called the Disphotic Zone, saying they were probably about 1000m below the surface. Nico Robin notes how quiet it is, completely different from a submarine, and how it reminds her of flying in the sky. A small pod of blue whales passes the ship. Brook cries, as they remind him of Laboon. Luffy thinks they look like Whitebeard and waves to them. While this is happening, the Caribou Pirates's ship is quickly approaching Thousand Sunny from behind, being towed by a shadowy sea monster. Caribou is heard rallying the crew, getting them ready for their upcoming attack. Caribou appears on his ship's deck, saying he wants to make their names known to the world by taking the Straw Hats' heads to display to the world.
Back on the Sunny, Usopp sees something gaining on them from the ship's six o'clock direction (the direction directly behind the ship). He thinks it is a sea monster and tells the crew there is a shadow heading directly toward them. Caribou's ship soon catches up and comes along side the Sunny. The ship looks like it is trying to ram the Sunny, but it is really trying to get the bubble roofs to touch. The Straw Hats wonder whose ship it is. Caribou leads the assault. At that moment, Nami recognizes the monster pulling the ship as Momoo, the pet sea cow of the Arlong Pirates. Momoo sees Nami, Sanji, and Luffy, remembering all three of them from the past. He remembers the beatings he received from Luffy and Sanji, and swims back the opposite way out of terror, towing the ship with him, but not before Caribou (and only Caribou) was able to make it onto the Thousand Sunny's deck. The Caribou Pirates still on the ship are completely dumbfounded as to what just happened. They also wondered why there was a sea cow attached to their ship.
Caribou, unaware he was left behind, commands his crew to introduce themselves by slaughtering all the Straw Hats. He immediately realizes that he is the only one of his crew on board the ship and immediately changes his tune. Franky picks him with one hand, Caribou begging him not to throw him out into the ocean calling them gentlemen, saying that human life is not something to be wasted, and that God is watching them. Franky, instead of throwing him out into the ocean, simply throws him against one of the side rails of the ship. Zoro asks if he was the captain of the ship that just went by. Caribou, realizing who asked the question, immediately denies this, saying he is just a human bullet for that crew, and that he is sick of being on that ship. He then asks the Straw Hats if it would be alright if he rode with them for a while. Chopper believes him, saying he might be a victim. Usopp, ironically, calls everything Caribou said a lie, which leaves Chopper surprised. Nami asks Caribou about why they had a sea cow pulling their ship. Caribou calls her a cutie-pie, asking her if she is "Cat Burglar" Nami. Sanji then intervenes, kicking him in the face and pinning him to a wall, saying he had no right to look at Nami with his despicable eyes. He is then about to tell Nami that everything is fine. However, before he could finish his sentence, he breaks out in another nosebleed. Usopp says that his nosebleeds are beginning to be a pain in the ass. Chopper suggests they rehabilitate him using low intensity pictures. Caribou explains that they had caught Momoo in the area, saying that having a sea monster pull their ship is the mark of a pro undersea traveler.
The scene momentarily switches back to the Caribou Pirates' ship, the crew trying to command Momoo to stop and asking him what happened. Coribou then says that Caribou will save them for sure.
Back on the Sunny, Luffy asks if anyone sees a sea cow anywhere, and then Nami asks him what he is planning. Usopp then notices that the ship had shifted direction and asks Nami if it was ok. The scene momentarily changes before her answer. Luffy is poking a tied up Caribou with a stick. Caribou is reworking his plan. Since he can get out of the ropes any time he pleases and since he cannot fight nine people at once, he thinks it would be best to observe the crew and wait for a chance to kill them one by one. He starts to come up with another idea but is cut off. Nami then said that the slight shift was fine, as they are supposed to be southwest of the log pose needle. Luffy asks if it would not be faster to travel in a straight line while Brook is saying he is starting to feel cold, and is about to say that he does not have skin but Chopper beats him to it, by saying that is amazing since he does not have skin. Nami explains that if they continue straight, they would just be swallowed up by currents and could hit a volcano or sea mountain before finishing their descent. Usopp says that the Grand Line's currents have always posed a hazard of some kind, and asks if there is a certain one to take.
Nami replies that there is only one, and tells everyone to put on a coat, saying the journey will get a lot colder. Zoro asks if they were going somewhere cold, saying he thought they were going to the deep sea. Franky corrects him, saying that the water in the deep sea is cold. Just like the bathtub, the water is colder on the bottom than it is on the top. Brook then adds that the colder levels have what are called Deep Currents, large ocean currents normally invisible to the naked eye that move in a completely different pattern than surface currents. Franky adds that all the currents on both levels eventually connect and reconnect with each other independent of depth or whether or not they are traveling east to west, comparing it to a giant dragon. He also added that the currents travel the whole world without ever breaking. Brooks then says that he has heard that the Deep Currents move very slowly across the ocean floor, and that once someone is down that deep, it could take nearly 2000 years to return to a part of the sea that gets sunlight. He then adds that there are many legends relating to the currents about monsters, curses, and ghosts. This excites Luffy and frightens Chopper. Nami says that to get there, they need to ride a descending current that takes them from a surface current to a deep current. Chopper asks how they find a spot like that. Robin replies that like a bath tub, cold water will move downwards. Usopp then concludes that they are going to a place where sea currents lose their warmth. Nami describes the Red Line as a group of adjoining islands, each with a different season. They are heading toward a climatic zone where there are winter islands, saying the area fulfills another prerequisite for currents to go down.
Luffy, suddenly interested, asks what the prerequisite is. Nami says it is about salinity, or the salt content found in sea water. Luffy said he played salinity all the time and Zoro said he always wanted a fire-sword. Luffy then asks Zoro how far he was sent away. With the two distracted, Usopp and Chopper ask Nami to continue explaining about salinity and ocean currents. Surprised by their enthusiasm, Nami goes on, explaining that a necessity for a downward current is the presence of cold, heavy water and that in cold areas, sea ice exists. Before she can continue explaining, Luffy, Zoro, Usopp, and Chopper are all seen drinking tea, saying simultaneously "So, it's a mystery current, huh?" This angers Nami to no end, and she yells that they should have just been quiet and rode from the start. Brook offers to sing. Franky then calls to her from the bow of the ship, saying "it" has come into view. Everyone goes to the bow. Luffy asks what the current looks like. Franky, in response to seeing it, says "Mother nature really knows how to get things done". Sanji asks what it is. Nami replies that it is called a Downward Plume. It is just like a giant undersea waterfall. Everyone looks at the Downward Plume in shock and awe. Robin comments on its massive flow, wondering how many hundreds of meters it was in diameter. Brook said that hearing it and seeing it are two completely different things. Usopp was scared that since it was too dark for him to see the bottom, assuming that the ship would crash and break up on the ocean floor. Nami questioned the current's safety, while Franky assured her that a ship born from Adam Wood could survive anything.
Caribou, who had been quiet the whole time, stands up and tells the Straw Hats to turn back immediately, saying that something was bad. Zoro, looked at him having forgotten he was there. Luffy says he sees a monster at the bottom. Caribou states he has never heard of that type of sea monster living in a place like a Downward Plume. He described the creature as such: "I've never heard that that thing was living in these waters!!! ......!!! That legendary demon that roams the vast ocean, never tiring of the slaughter it leaves in its wake...!! The enemy of all humanity!!!" The monster in question is a kraken, a giant, mythological octopus. Franky adds that it's crushing all the ships in its tentacles and that it must have taken all the ships that had set out in the past few days. While Usopp is commenting on its gargantuan size, Chopper is foaming at the mouth out of shock. Caribou said it must have been preying on ships trying to ride the downward current and that he had never heard of anything like this happening before. He begs Luffy to turn back, telling him to wait a few more days. Luffy simply calls him noisy and tells him to shut up. The chapter ends with Luffy saying he is going to make the kraken his pet.
- The colorspread of this chapter is a reference to the story Town Musicians of Bremen from Grimm's Fairy Tales.
- Momoo reappears. He seems to have been captured by the Caribou Pirates near the Sabaody Archipelago.
- Deep sea currents are further explained.
- Sanji has to have another blood transfusion due to his continued nosebleeds. In addition, a method of rehabilitation is suggested to help with Sanji's severe condition.
- Chopper ruins Brook's attempt at one of his bodily jokes, this one being about his skin, leaving the latter momentarily depressed.
- Franky grabs Caribou and Sanji kicks him in the face even though he is supposedly a Logia user. However, Caribou deliberately refrained from using his Devil Fruit powers to hide his ability as he is biding his time.
- Luffy claims that he can tame the kraken. This is the second time he has claimed that he could tame a dangerous creature, the first being the Cerberus on Thriller Bark.
- In 2019, One Piece Editor Nairo revealed at a Knowledge Quiz event that Oda had drawn Luffy picking his nose and having placed a booger on Usopp's arm with the hopes that a fan would notice and point it out in an SBS.
- Luffy is seen picking his nose on page 12 while Nami explains the deep currents, and having placed a booger on Usopp's arm, with Luffy's finger can briefly be seen when Usopp marvel at Brook's knowledge. On page 14, Usopp is shown with a suspicious dot on his arm, unaware of Luffy's deed.
- In the corresponding anime adaptation in Episode 524, Luffy is seen picking his nose as well, however none of the following events are shown.