On April 26th, 1986, the Soviet Union (and the world with it) changed forever. At 1:23 MSD, Reactor 4 of the Chernobyl Nuclear Power Plant in Pripyat, Ukraine suffered a catastrophic meltdown, and a disaster of catastrophic proportions followed.

You are a liquidator, or one of the many thousands of civil & military personnel sent into the accident site to clean up & reduce the consequences of the disaster. (It's encouraged to make up your own scenario as to what you're tasked to do)