In 2147, two years after the Mars Base fiasco, the Union Aerospace Corporation drove the Expendable Resource Reconnaissance Bill through World Government. Primarily, the bill was intended to avoid further waste of valuable resources when returning to the sealed off facility. The average space marine cost over two million dollars to train and equip - each lost marine represented a serious drain on UAC resources. The ERRB solved this problem by ensuring a steady supply of low cost resources for missions with high casualty rates from maximum security prisons. The most valuable assets came from the Glasshouse: marines who had refused to bear arms. These prisoners were highly trained, psychologically stable and displayed strong survival instincts. And, of course, if they never made it back, they wouldn't be missed. You're going back to Mars. But this time, your shotgun has only got rubber bullets in it. Conscientious Objector: killing is for wimps.
Zip file containing the Conscientious Objector mod. Unzip into your Doom directory and use the Mod option at the bottom of the main Doom menu screen.