You are to consult with Chief Sonu about finding a suitable island to use as bait for Bismarck.
This quest is available if you have completed the quest Black and the White.
Starting the Quest
Talk to Alphinaud in the Sea of Clouds to begin the quest.
Speak with Sonu Vanu
Go southeast to the wooden stairs between the Zundu Sell-it and the Zundu Fix-it, and go up. Then go south to the quest icon on the map and talk to Sonu Vanu.
Speak with Cid
Go north to the wooden stairs and go down, then go north and follow the dirt path that winds through the area. It will lead you to some more wooden stairs, and at the top you will find another dirt path. Go along it until you can go southeast to the quest icon on the map. Talk to Cid to proceed.
Speak with Wedge
Talk to Wedge, and say Yes if you are ready.
The Limitless Blue (Hard)
Use the Duty Finder to enter The Limitless Blue (Hard). Please see the The Limitless Blue (Hard) Guide for a walkthrough.
After you complete the duty in The Limitless Blue and exit the area, there will be a cutscene. This one is important, so it is recommended to watch it either now or via the Unending Journey in an inn room.
Speak with Alphinaud
Talk to Alphinaud to complete the quest.
Unlock Flying in the Sea of Clouds
"Bolt, Chain, and Island" is the final quest for unlocking Aether Currents in the Sea of Clouds. If you have completed the other Aether Current quests and attuned to the Aether Currents in the field in this area, which you can find using the Aether Compass in Main Menu > Duty > Collection in this area, you will be able to fly in the Sea of Clouds.
Next Main Scenario Quest
After you complete the quest, talk to Alphinaud to accept the next quest: A Difference of Opinion. There will be a cutscene. This one is important, so it is recommended to watch it either now or via the Unending Journey in an inn room.