Marine Rookie Arc

The Marine Rookie Arc (海軍超新星（ルーキー）編) is the thirteenth filler arc and the fourth filler arc after the timeskip.

After running out of food on the way to Whole Cake Island, the Sanji Retrieval Team sneaks into a Marine base to steal some.

Summary
The Sanji Retrieval Team was famished due to Luffy wasting all their supplies, but they came to an island that Nami had seen on the map in Zou. They prepared to look for food there, but Pekoms warned them that the island, Fron Island, was home to a Marine base that kept an eye on Big Mom. Luffy, Nami, Chopper, Carrot, and Brook then headed to the island on the Shark Submerge III while Pedro and Pekoms stayed behind to guard the Sunny.

On Fron Island, Vice Admiral Prodi watched as a ship carrying reserve forces drew near to the island. However, the ship suddenly erupted into chaos as Grount, a 17-year-old captain, started attacking his comrades. Grount easily overpowered his comrades with his tremendous power and reported to Prodi that they were all weak, angering the vice admiral. Prodi took Grount to his office and called Garp to find out why Grount was transferred here. He discovered that Grount requested to be stationed here in order to take down Big Mom, and dismissed that dream before angering Grount further by belittling Aokiji, his former commander.

The Straw Hat Crew knocked out a group of Marines and stole their clothing, and Brook went back to the Shark Submerge III while the rest prepared to raid a food storage building. However, Luffy and Carrot went to a cafeteria, where they were served food. Grount went to the cafeteria as well, where he reunited with his old comrades under Aokiji, Bonam and Zappa. Nami and Chopper then entered the cafeteria, and Carrot called Nami's name, causing Zappa to notice her. Zappa approached Nami and proposed to her, but Nami ran away, leaving him sullen again. Meanwhile, Prodi found out about the Marines the Straw Hat Crew attacked and activated an alarm as he and his forces searched for the intruders. When Luffy caused an altercation in the cafeteria by taking an entire bowl of curry, Prodi spotted him, and ordered his men to battle against the Straw Hat Crew. As this happened, Nami and Chopper ran to the storage building to get more food rations, while Brook, Carrot, and Luffy were all surrounded by Marines. An excited Grount then prepared to fight Luffy.

Luffy was uninterested in fighting and tried to leave, but Grount lunged at him and Luffy countered his daggers with the curry ladle. The two dueled, but Luffy got the better of Grount by slamming the bottom of the ladle into the Marine's face. Bonham then stepped in and attacked Luffy with rapid Rokushiki moves, but Luffy managed to avoid all of them and he tried to leave the room. Bonham blocked the exit and activated Tekkai, but Luffy activated Gear Second and blew him away. Luffy and Carrot ran down the hall, but were approached by Zappa, who tried to woo Carrot. Zappa got jealous of Luffy and tried attacking him, but Luffy blocked his attacks before Carrot knocked him down with Electro. Luffy and Carrot were then approached by Brook's detached soul, and they raced to reunite with Nami and Chopper with Grount, Bonham, and Zappa in hot pursuit. Brook led them to a dead end, and Grount and Bonham cornered the trio. However, they argued over who would get to take down the intruders, allowing Luffy to punch a hole in the wall and allow him and Carrot to escape.

Luffy and Carrot raced to the food storage facility where Nami and Chopper were, and they started gorging on food. They reunited with Nami and Chopper, but the group was quickly confronted by Grount, Bonham, and Zappa. Chopper took on Bonham in combat, but was initially overpowered by Bonham's Rokushiki. However, he managed to block Bonham's attacks with Guard Point, before activating Heavy Point and blowing Bonham away despite him using Tekkai. Zappa continued to try and woo Nami and Carrot, and attempted to take down Luffy so he could have the ladies to himself. Both Carrot and Nami struck him with electric attacks, but Zappa refused to stay down as his swords caught fire and he unleashed a giant fiery whirlwind at Luffy. However, Luffy redirected the attack at a giant pot of water above them, causing it to crash down on Zappa and flood the facility. The Sanji retrieval team was washed outside, and just as they tried to escape they were confronted by Grount. Luffy brawled with Grount and eventually got the upper hand, sending a bloody Grount sprawling on the ground. As Bonham and Zappa returned and tended to Grount, the team tried to escape once more, but when they got to the coast, they were confronted by Prodi, who was determined to take them down in order to avoid getting demoted.

The team tried to escape onto the Shark Submerge III as quickly as they could, but Prodi shot it with his bazooka before confronting them. The team managed to dodge his shot, and Luffy confronted him as the others confronted his troops. The Marines managed to nullify Nami's and Carrot's electric attacks with rubber shields, but were easily defeated by Nami and Chopper physically. As Prodi shot his bazooka at Luffy, Grount, Bonham, and Zappa watched the effects of the battle from far away. Bonham and Zappa wanted to go and fight, but Grount sullenly stayed where he was, calling himself weak. Bonham reprimanded him, saying to remember what Aokiji had taught them, but Grount stayed down and Bonham and Zappa departed from him. At the coast, Luffy defeated Prodi with Jet Bazooka, and Bonham and Zappa came to confront the pirate in a rematch.

Meanwhile, Grount remembered meeting Aokiji. When he was very young, he had a monstrous left arm that was extremely large and covered in red fur. He was unable to control its power, causing him to destroy an entire island and making him extremely distressed. He was appraoched by Aokiji, and unwillingly attacked him as well, but Aokiji froze his left arm and took him in. There, Grount received a metallic attachment that covered his left arm and limited its power. He trained under Aokiji with Bonham and Zappa, but was never sent out to battle. Aokiji told him he was not yet prepared, and taught him that he would need to master his power in order to take on powerful enemies. Back in the present, Grount headed toward the coast as Luffy took down Bonham and Zappa. Brook revealed that he and the Submerge had survived Prodi's attack, but before the pirates could escape, Grount ambushed Luffy.

Grount prepared to unleash his full power, and he exerted his left arm, causing his metallic attachment to shatter and reveal his monstrous arm, which he now had full control over. Grount managed to overpower Luffy with his left arm, but Luffy was able to clash evenly with him by activating Gear Third. Grount hit Luffy high into the sky, but Luffy prepared to attack him with Thor Elephant Gun. The two then clashed with their giant fists, and the effect of their clash created shockwaves that rocked the coast. Eventually, however, Luffy overpowered Grount and sent him flying into the ground, defeating him. The Sanji retrieval team then finally made their escape, and Grount regained consciousness soon afterwards. Hearing about what happened, he simply laughed and noted that Luffy's performance was worthy of his bounty, promising with Bonham and Zappa to win the next time they met the Straw Hat captain. The Straw Hats departed from Fron Island on the Thousand Sunny, and to Nami's shock, the rations they had stolen were depleted in a matter of 10 minutes. Pedro and Pekoms had eaten most of them due to being left starving on the ship, but even the crewmates who had eaten at the Marine base had feasted. As the team sailed to Whole Cake Island, Luffy looked back at Fron Island and Grount, who was looking at him.

Trivia

 * This is the second filler arc in which Luffy and his group infiltrate a Marine base, the first being the G-8 Arc.
 * However, unlike the first one, they do so intentionally this time.