Chapter 603 is titled "Keep It In Your Heart".
"Sanji And Some Cute Squirrels Make A Pie", requested by NSC Hayao.
The Straw Hat Pirates are finally on their way to Fish-Man Island. After Nami explains the properties of a coated ship, Franky reveals why Hatchan was not there to guide them. As it turns out, the Marines discovered the Thousand Sunny and tried to claim it, with Duval and Hatchi sustaining heavy injuries in the ensuing battles. Franky also reveals that before Bartholomew Kuma was fully converted into PX-0, he made a deal with Dr. Vegapunk to allow him to include a mission of his own into his programming, to protect the ship of the Straw Hat Pirates until the day one of their members returns. As such, the Straw Hats are deeply indebted to Kuma for not only saving their lives but for protecting their ship as well. The chapter ends with the Caribou Pirates in pursuit of the unaware Straw Hats. It is also revealed, in a flashback of Rayleigh's, how the Roger Pirates started and that the straw hat that Luffy wears upon his head once belonged to Gol D. Roger.
The chapter starts out in the "no man's land" on Sabaody Archipelago. Caribou, Coribou, and their crew somehow escaped the battle and are holding the Imposter Straw Hats (save Black, Cocoa, and Fake Chopper) captive. The fakes beg for their lives as Coribou and the Caribou pirates dig holes, preparing to bury the impostors alive. Caribou reveals that he never intended to follow the Straw Hats and had planned to kill them from the inside and is very angry that he met fakes instead. The impostors continue to beg for mercy and Caribou then grabs fake Sanji and starts to suffocate him with his Devil Fruit powers. Fake Sogeking tries to stop Caribou with a bazooka but fails, as the shot is just absorbed into Caribou's body. The impostors are terrified, learning that Caribou is a Logia. Caribou then orders his crew to bury the fakes and happily states that he managed to confirm the real Straw Hat Luffy.
Sentomaru is then seen talking to his superiors about the Straw Hat crew's escape and their newly acquired strength with Demaro Black lying on the ground, tied up and unconscious. Shakky and Rayleigh are then seen having a conversation at the coast where the Straw Hats departed. Rayleigh then thinks back to the day when he first met Gol D. Roger. This flashback also reveals that Luffy's straw hat once belonged to Roger. Rayleigh then states that Luffy has taken another step towards becoming a man worthy of that hat he wears.
While the Thousand Sunny is sailing underwater, the crew marvels at the site of the underwater world. Luffy and Zoro attempt to catch some of the fish only to be stopped by Usopp and Chopper. Sanji asks Nami about what the crew needs to know about the coating. As Nami starts to explain, Sanji got another massive nosebleed just by seeing Nami's body move. Sanji propels himself out of the coated ship and had to be grabbed and pulled back in by Luffy. Chopper immediately starts treating Sanji while Brook shows his concern for the cook, wondering what will happen when they see the mermaids on Fish-Man Island. Nami then explains that a coated ship has all the same properties of the bubbles on the Sabaody Archipelago, where they will expand until a certain point before letting an object go through the bubble. If it gets too many holes, it would pop. Luffy and Zoro try catching fish again only to be stopped by Usopp and Chopper once more. Since Sanji is out of commission, Luffy and his crew start eating the special lunch boxes given to Luffy by Boa Hancock. Franky explains to the crew how the Thousand Sunny was kept safe for two whole years. He stated that during a battle with the marines, Hatchan and Duval suffered serious injuries and Bartholomew Kuma took over as the ship's protector. It is revealed that Kuma was allowed by Dr. Vegapunk to include a single mission of his own into the programming, to "protect the ship of the Straw Hat Pirates until the day one of their members returns". The Straw Hats agree that they are indebted to Kuma. The chapter ends with the Straw Hats being unknowingly followed by the Caribou Pirates.
- Caribou is revealed to have a Logia ability.
- The faces of Fake Sogeking and Fake Franky (who was wearing shades) are fully shown, and the real name of Fake Sanji, Drip, is revealed.
- Apparently all the pirates recruited into the Impostor Straw Hats (except for Caribou, Coribou and their crew) were captured by the Marines. Coincidentally, just like the Supernovas from two years ago, Caribou and Coribou manage to elude the marines.
- The Caribou Pirates reveal that they were planning on killing the Straw Hat Pirates from the start and that this was the only reason they considered joining the crew.
- The Pacifista PX-5 and PX-7 are revealed to be older models of the Pacifista that appeared at Sabaody two years ago.
- A young Roger is seen for the first time. In his youth he bore an uncanny, near-identical resemblance to Luffy.
- It is revealed that Roger wore the straw hat that Shanks gave to Luffy.
- It is revealed that Bartholomew Kuma was allowed by Dr. Vegapunk to include one mission of his own before losing his personality, in which he was secretly programmed to protect the Thousand Sunny until one of the Straw Hats returned.
- For the first time, Chopper and Usopp are seen beating up Luffy and Zoro much like what Nami usually does.
- Ship coating is further explained.
- Sanji's nosebleeds, while previously used for comedic effect, have been recognized by the crew as a serious problem due to how easily invokable they are now. Because of his continued nosebleeds, Sanji has to have a blood transfusion.
- This is the first chapter of the Fish-Man Island Arc.