A world full of radioactive gas. The Underground pnp takes action after the nuclear war. When you are in a city the only thing that survived are monsters of the mutant class.
Because of the toxic gas you need a gas mask with filters. There are many different types of filters, Some are larger, some are refillable. You can craft them with normal filters and some other stuff. (See crafting)
In game the time ticks for your character. The filters run out. Because role playing is really slow in real life the time in real life ticks slower. If you have 5 minutes of filter durability you start a timer of 15 minutes (3 times of your filter durability)
If your filters run out you die. Yeah it's hard!