From the introduction to the core book:
This is a roleplaying game that is designed to be run with no preparation in a single game of two to four hours. It is inspired by the cinematic soldiers versus aliens genre – that means Aliens above all else.
Every game session has the same basic setup – a squad of marines is heading out to investigate an isolated colony (or where ever). Contact has been lost and Colonial Defense is suspicious. Inevitably, there's a hostile force waiting there and it's up to you guys to save the day or die trying. And do it with style.
To make each game different, the mystery is invented as you go along. All players get an equal say in what actually occurred.