From the back of the DVD case:
Earth is still rebuilding from the devastation from the 30 years war. There's a power struggle between the Corporations and the Earth Government and you're right in the middle. The Earth government has contacted you, a mercenary known as a Raven, to do battle on their behalf. You own your own Armored Core Unit and you're always looking for another chance to buy upgrades and new weapons. Do you want to tell the Earth Government that the Corporations have also been calling for your services?
Didn't think so. Let them handle the political stuff, you're just interested in getting the best missions from the highest bidder - it doesn't matter to you who signs the check, right Raven?