Keelhaul Key is the main area in which Chapter 5 of Paper Mario: The Thousand-Year Door takes place.
Similar to Lavalava Island from Paper Mario, Keelhaul Key is a southern island disconnected from all kind of society of the outside world. During the events of Paper Mario: The Thousand-Year Door, Mario learns that the next location of a Crystal Star is in the southern island of Keelhaul Key. Mario will soon find himself captain of a whole crew, which consists of Flavio, Pa-Patch, Four-Eyes, two Toads, and some Bob-ombs. While on the trip to Keelhaul Key, the ship, the S.S. Flavion, is attacked by Embers and finally sinks.
Luckily for Mario, the ocean washes the whole crew up to Keelhaul Key itself. Mario will now have to face the dangers of the jungle in order to save his lost crew mates and find a way into Pirate's Grotto, in which the Sapphire Star lies. With the help of Flavio's Skull Gem working as a kind of key, Mario makes it inside. At the end of Chapter 5, Mario faces off against Lord Crump in his ship at the shore of Keelhaul Key. After completing the Chapter, the player may return to the southern island on Cortez's Ship, Black Skull, whenever he or she wants. Cortez's Ship will be in Rogueport at all times. There is also a pipe in Rogueport Sewers which Mario can take to Keelhaul Key.
Keelhaul Key is the only place in which Mario can find Keel Mangos, used for a variety of recipes. Additionally, in Keelhaul Key, Mario's last compulsory party member, Bobbery, joins him. Mario can also find several badges and items on the island.