The lesson taught by Socrates' "Allegory of the Cave" is simple: you can't go home again. Once you've left the safety of your quiet, underground home, the things seen and done in the outside world change everything. Most of all, they change you.
Imagine, then, being forced from the only home you've ever known and staggering out into the red sunlight of a world burned by nuclear fire. Inside the vault, you were surrounded by the happy images of an optimistic society. That's all long gone. Humanity's final war reduced civilization to a wasteland of isolated outposts, bloodthirsty raiders and horrific mutation, and this is your home now. Basic survival is your priority. Food. Water. Shelter. Weapons. You must make your way through this changed Earth, learning as you go and maybe, hopefully, changing things a little for the better.