The "Water 7 Saga" (ウォーターセブン編,Wōtā Sebun Hen?), also known as the "World Government Saga" or "CP9 Saga", is the fourth saga in One Piece. After their adventure in Skypiea, the Straw Hat Pirates land safely back in the Blue Sea, only to find themselves confronting Foxy the Silver Fox in the Davy Back Fight, and soon after, Aokiji, one of the Navy's three Admirals. Narrowly escaping Aokiji, the crew reaches Water 7, a city of carpenters, and begins searching for a shipwright to join the crew.
They soon fall into trouble though, making enemies with both the Franky Family: a ship dismantling group, and the Galley-La Company: a shipbuilding company which employs some of the top carpenters in the world. They also learn that the Going Merry is dying, and because of this, Luffy decides to get a new ship. Usopp disagrees with the decision and leaves the crew. Meanwhile, CP9 of the World Government captures Robin by her will and flees to Enies Lobby, while the crew attempts to get Robin back. Finally, they get her free and get a new crewmate: Franky, who is their shipwright. As they are leaving, Usopp apologizes and is welcomed back into the crew, and so the Straw Hat Pirates are ready to set sail with a brand new ship and shipwright, Thousand Sunny and Franky, along with Robin and Usopp back in the crew.
This is currently the third longest saga in the manga, after the Dressrosa Saga and the Four Emperors Saga.
This is the only saga in the first half of One Piece to not introduce a new Warlord of the Sea.
It is during this saga that the Straw Hats' need for a shipwright, as well as a new ship to replace the Going Merry, becomes clear. This saga is the last saga to feature the Going Merry and was the first saga to feature the Merry's successor, the Thousand Sunny.
This saga involves several members of the Straw Hats leaving the crew temporarily. During the Long Ring Long Land Arc, Chopper is forced to leave the crew during the Davy Back Fight between the Straw Hats and the Foxy Pirates (in the anime, Robin is forced to leave during the Davy Back Fight as well), though the Straw Hats later win him (and Robin in the anime) back. During the Water 7 Arc, Usopp leaves the crew following his and Luffy's fight over the Going Merry, while Robin is forced to leave due to the threat CP9 and the Buster Call pose to the Straw Hats. Robin later rejoins the crew during the Enies Lobby Arc, while Usopp officially rejoins (as he used his Sogeking alter ego during the later part of the Water 7 Arc and the whole of the Enies Lobby Arc) during the Post-Enies Lobby Arc.