Marine Rookie Arc

The Marine Rookie Arc (海軍超新星（ルーキー）編), or the Fron Island Arc, 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 saw 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 Prody 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 Prody that they were all weak, angering the Vice Admiral. Prody 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 Hats 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, Prody found out about the Marines the Straw Hats 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, Prody spotted him, and ordered his men to battle against the Straw Hats. 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.

Trivia

 * This is the second filler arc in which Luffy and his group infiltrate a Marine base, the first being the G-8 Arc.