When the Colony on Hayden's world went black, a team was sent to investigate. It was supposed to be a training mission : skip in, find out the Casimir Transmitter had gone dead, report back and wait for resupply from the Fleet. By the time the only surviving member of the team made landfall, it was spectacularly clear that this wasn't a training mission.
Now Sgt Sorilla Aida has a job to do. She has to gather the local survivors, recon the enemy, train a militia, and take the war right to the enemy's doorstep.