Marine Rookie Arc

Slotted between Zou and the Big Mom saga, this is counted as the anime's thirteenth invented storyline and the fourth such detour since the two-year timeskip. None of it appears in Eiichiro Oda's manga. Two points make it stand out among filler: it is the newest one not made to advertise a film, and its trio of new antagonists, Grount, Zappa, and Bonham, were the first characters ever designed by Midori Matsuda, who would go on to become the show's lead character designer from Wano onward. A break-in at a Marine facility had happened once before, back in the G-8 detour, but here the crew chooses to trespass on purpose rather than stumbling in by accident.

Their supplies squandered by Luffy, the hungry group stops at Fron Island, which Pekoms identifies as the site of a Marine post keeping watch on Big Mom. Leaving Pedro and Pekoms to mind the Sunny, Luffy, Nami, Chopper, Carrot, and Brook slip ashore aboard the Shark Submerge III. Inside, Vice Admiral Prodi is losing patience with a seventeen-year-old captain called Grount, whose raw might lets him flatten his fellow soldiers and who came to this posting hoping to bring down the Emperor, an ambition Prodi ridicules. Wearing stolen uniforms, the pirates split between a storeroom and a cafeteria, where Luffy's gluttony and a lovestruck swordsman named Zappa harassing Nami blow their cover and set off an alarm.

A chase and a string of skirmishes follow as Grount, Bonham, and Zappa, all former pupils of Aokiji, corner the crew. Luffy blocks Grount's daggers with a curry ladle, Chopper turns back Bonham's Six Powers, and electric strikes from Nami and Carrot wear Zappa down. Prodi finally confronts them at the shore, only to fall to a Haki-charged bazooka. A flashback then explains Grount: as a small child he possessed a huge, red-furred arm he could not command, one that leveled an entire island until Aokiji froze it, took him in, and bolted a metal restraint over the limb while training him to control it. In the present, Grount snaps that restraint off and attacks at full strength, but Luffy answers with Gear 3 and slams him into the earth.

Being non-canon, the detour leaves the main plot exactly where it found it and simply nudges the crew back onto their heading. Grount wakes soon after his loss, laughs the whole thing off, and declares that Luffy's showing lived up to the price on his head, and he vows with his two comrades to prevail in any rematch, while the three renew their faith in what Aokiji taught them. Back aboard, Nami is dismayed to find that every stolen ration is already gone within minutes, wolfed down both by the famished pair left on the ship and by those who had eaten at the base. As the Sunny departs, Luffy glances back at Grount, and the two watch one another until the island fades.