Premise: A group of four stumble upon a seemingly abandoned diner, small town, or research facility. Once within they are beset by a single enemy type: either ghouls, mutants or deathclaws. With each wave, the number of these creatures will increase; however new areas will unlock bearing weapons and gear. After a set number of rounds you will be offered an escape; however if you stay and complete a particular task (repair a generator, save an NPC, board up a dwelling with wood) you stand to reap greater rewards.
Regardless of whether you live or die, you will be given a score sheet when you exit the area--one showing the level you progressed to and how well you did in comparison to other fonline players.