London has fallen. But fallen where? Try under the Earth itself. This is the basis of Sunless Sea. Set in the world of Fallen London, in a gothic steampunk-esque universe, you must try to survive when you're facing things that are straight out of the nightmares of others. But you must steel yourself, for your mind might be the first thing to go if you're not careful in this dark and dreary world.
You are going to be the Captain of your own steamship. You'll customize them to be the perfect captain, or, to be the kind of Captain you want them to be. Be mysterious, be known, be skilled, be a novice, whatever you feel is right. Just know that your choices will affect how you play.
Then, try and get a crew. Can't get a crew? Sail yourself? Sail to where? Sail to anywhere on the map! The world of Sunless Sea is mostly unknown, but there are things out there for you to find...or to find you. While there are human colonies, there are also horrible sights, monsters that will try and kill you, your crew, and your ship. So you must be ready, you must be wary, and if all else fails, you must be ready to die.
Your journey doesn't end if you fall though, it just gives you experience for the next journey. Be outfitted to handle anything that comes your way. Complete missions so you can get a better ship, better crew, and learn to not fear what lies in the darkness of this fallen world.