Water 7 Arc

Numbered fifteenth in the series and forming the middle stretch of the Water 7 Saga, this story brings the Straw Hats to a sprawling city on the water where they hope to mend their failing ship and maybe gain a new crewmate. Trouble finds them fast through a gang of ship-breakers called the Franky Family, plus a conspiracy that runs deep inside the Galley-La Company. Emotional rupture defines the arc: Robin slips away under strange circumstances, and a savage disagreement over whether to keep the Merry splits Luffy and Usopp.

It is also a heavy dose of world-building, unveiling the covert government unit Cipher Pol 9, the ancient weapon Pluton, and Franky's sorrowful history bound to the shipwright Tom and the fallout of Roger's execution. The Sea Train Puffing Tom and the yearly flood tide Aqua Laguna lend the city its flavor, and the arc ends by hurling the crew toward Enies Lobby to win back their vanished friend.

Healed after their run-in with Aokiji, the crew follows a giant frog onto submerged rails and meets the stationmaster Kokoro, who points them toward the city and the shipwright Iceburg. Once they cash in their Skypiea gold, the Franky Family robs them, and they get the crushing verdict that the Merry's shattered keel puts it past saving. Luffy decides to buy a new vessel, and when Usopp protests that casting off the Merry is like deserting a friend, the rift boils over into a duel. Luffy wins but hands the ship to Usopp, who leaves the crew.

The city then explodes into panic as Iceburg is shot and names Nico Robin among his assailants. The masked attackers prove to be CP9, agents who faked the attempt to pin it on the pirates while hunting the Pluton blueprints Iceburg holds from Tom. A long flashback exposes Franky as Cutty Flam, Tom's apprentice, whose own warships were turned against his master and who was thought dead beneath the Sea Train. With Robin surrendering herself to shield the crew and Franky seized, the agents board the Puffing Tom for Enies Lobby, riding out into the fury of Aqua Laguna.

Unwilling to be abandoned, the crew commandeers the prototype train Rocketman under Kokoro's hand and strikes a three-way pact joining the Franky Family with the Galley-La foremen, all racing for Enies Lobby to save Robin. Sanji stows away on the Puffing Tom and, alongside a masked Usopp now styling himself Sogeking, fights car to car through government agents to reach her. The arc cuts off mid-pursuit as both trains bear down on the judicial island.

Its events permanently redirect the crew. Franky's debut sets up his eventual role as shipwright, the doomed Merry steers the crew toward the future Thousand Sunny, and CP9's Six Powers would later spark Luffy's Gear 2. Robin's coercion by the government, held under threat of an island-flattening Buster Call, becomes the driving grief that carries the crew into the Enies Lobby Arc.