What happened?

Plot point? Nothing, really.

They all live in huge underground bunker. With no elevator and must run up the stairs between floors. They try to run society with police and court. Once in a while they throw out people outside to clean up camera that shows outside picture from one position. shows wasteland. Those people die indicating that air is still bad.
They had a revolt some time ago after which they burned all books or something. Now no one knows why they live in bunker and what happened to the world. And they destroy every peace of relics from old life so that no one would know how old world looked like, its a forbidden thing.
In the end one woman gets out of the bunker and doesnt die. Camera shows us that there are like 40 other bunkers near by. Indicating other societies like this exists too. Cliffhanger, end of season.