The Whisky Peak Arc, also known as the Cactus Island Arc, is the eighth story arc in One Piece and the second arc of the Alabasta Saga. It involves the Straw Hat Pirates: Monkey D. Luffy, Roronoa Zoro, Nami, Usopp, and Sanji, together with new characters who will be known from the Baroque Works Organization, including Mr. 8, Igaram, Miss Wednesday, Princess Vivi, Mr. 9, Miss Monday, Mr. 5, and Miss Valentine.
This arc happens right after the Straw Hats set foot in the Grand Line. It takes place on Whisky Peak, an island that is home to so many cactus-shaped mountains that make up its outline. The central reason for this arc is basically the first stop of the Straw Hats within the Grand Line and their accidental embroilment in a bigger conflict involving Alabasta Kingdom.
The Whisky Peak Arc sets off when the Straw Hat Pirates finally reach Whisky Peak, only to be received in a manner quite opposite to what they expected: hospitality. Residents welcome them with feasting and a big celebration into which the crew, especially Luffy, goes head first. However, it later transpires that this kind reception is all part of Baroque Works's scheme—a criminal organization operating secretly. Zoro becomes suspicious and remains on his guard, only to find out that the townspeople are actually bounty hunters working for Baroque Works, tasked with eliminating pirates who enter the Grand Line.
During the night, he managed to defeat all of those catchers solitary, thus proving he is a great swordsman and rather powerful. It creates a bit of a problem with Mr. 8 and Miss Monday, both higher-up members in Baroque Works. In the fight, he finds out that their true identities are Igaram, the head of Alabasta's royal guard, and one of the princesses of Alabasta—Vivi. The two had infiltrated Baroque Works to uncover the organization's plans to overthrow the Alabasta Kingdom.
This only gets worse with the entrance of Mr. 5 and Miss Valentine, who are tasked with assassinating Vivi and Igaram for their treachery. The development thrusts the Straw Hats into the fray. First of all, Luffy and Zoro end up fighting each other due to a misunderstanding. This shows the comedic elements that One Piece tends to throw in around serious plot points. Eventually, the crew has Vivi's back: Luffy defeats Mr. 5, and Zoro defeats Miss Valentine.
As the arc proceeds, Vivi reveals critical data on Baroque Works and its leader, the Warlord Crocodile, whose true identity she has finally revealed. Setting up the larger Alabasta Saga, it sets out that the Straw Hats agree to escort Vivi back to her kingdom to prevent a civil war orchestrated by Crocodile.
The Whisky Peak Arc concludes with the Straw Hat Pirates having to flee with the island's princess, Vivi, and her duck, Karoo. It is Igaram who tries to act as a decoy, letting them go, but he is apparently killed by Miss All-Sunday, who is the vice president of Baroque Works. This lends some real urgency to the mission of the crew, further portraying the dangers of Baroque Works.
This arc's conclusion marks a turn in the One Piece narrative. It shows the general conflict of the Alabasta Saga and gets the Straw Hats involved in a far bigger political struggle, which actually expands the scope of the series' arc from individual adventures to a much greater tale with factions and nations pitted against one another. By forming this alliance with Princess Vivi, the Straw Hats gain one new but temporary crew member and join a cause that will pit their might and resolve against one of the most overpowering organizations in the Grand Line.