The four of you have survived The Nae Nae Swag Incident, quite possibly the worst thing to happen to society. Sean, the leader of your defunct civilization never recovered from the incident and it collapsed because of it, you managed to escape the world only to return to find the world in chaos. Have fun.
This is just the crashlanded scenario, nothing special here.
No, I will not tell you about The Nae Nae Swag Incident.
For added immersion, one character is to be named Sean