(even if you aren't vegan)
2 years ago, an incident occured at the Aperture Science Enrichment center...
The main computer system, GLaDOS, went haywire...
GLaDOS killed all the volunteers, all the scientists... everyone.
One day, a volunteer by the name of Chell defeated GLaDOS, and she was destroyed, and stayed dead.
Until 2009, when GLaDOS was repaired, and a newer model was created.
The Science Center was re-opened. Nobody had any doubts of the new GLaDOS. They thought they were all safe.
...But they were wrong.