So you hear the emergency radio signal. What does it say? Is it an asteroid headed to earth? Is it an alien space ship? Is it a supervolcano erupting? A nuclear strike? Pick a good disaster, either country-wide or world-wide. I’m going to pick the “CERN super-collider was testing something and they accidentally opened a portal to another dimension” one (think in terms of the video game “Half-Life” if you’ve played it).
You don’t need to know hard science, you can have the radio announcer say “an accident in Switzerland at the CERN facility has caused mass death in a two thousand kilometer radius” because let’s face it, radio and news people are only going to give you details once the event is over and everyone has had time to study it.
Okay, so I’m driving along, and the EBS (Emergency Broadcast System if you are American) goes crazy. The announcer tells you barely anything useful other than get to safety quickly (you can google the EBS warnings and listen to some for ideas on how they are announced).
Now we need a major plot device. The CERN boys opened a portal to…somewhere. Hell? The 9th Dimension? A parallel earth in another universe? The 34th Century? I’m going to say they opened a black hole that somehow has grown too large and is now sucking the earth into it.
As I was driving home, the EBS signal blared on the car’s radio. The announcer said we should take shelter immediately as a national disaster was happening. It becomes a madhouse getting home, people are starting to panic within minutes. I finally get home and turn on the news. Drone and spy plane images and even satellite images are showing a 600km wide hole to…nothing, and it is growing about two kilometers an hour. Some scientists think the hole will grow faster as it eats more matter from our planet. Now everyone is going crazy. Those living near it in Europe are basically dead already, but they’ll do anything in a panic to try and survive. They are like rats in a cage that has been put in a fire.