Humanity was nearly wiped out in some massive catastrophe, though the rumours as to what that was, vary. The remaining humans, created a super city, surrounded by walls taller than the buildings themselves, and topped with a specialised dome designed to filter the air and sunlight into something survivable. The upper class all live in luxury, with high quality tech made readily available. The lower class though, struggle to survive in the harsh conditions, forced to do heavy, dangerous, and dirty work to survive. Any who complain are quickly told that they should be grateful for the lives they do have, as life in the city is the only thing there is, the outside too inhospitable for people to survive.
It is as you are searching the very furthest border of your section of the city, looking for anything that can help you and your group survive another week, that you find the breach. It takes some careful climbing, but you manage to get down to where the breach sits, so very well hidden. To your shock, you find no wasteland or toxic earth, but instead the ruins of what once was, now reclaimed by thriving nature and animals, the air pure and clearer than you have ever felt.