when europa-1, the flagship of the planet defense corps, is attacked by an unknown yet lethal aggressor of apparently extraterrestrial nature, the planet defense corps call in zero tolerance, an elite strike squad of five speciality-trained commandos. a recording of the last transmission from europa-1 reveals extensive fire damage to the warship, almost total casualties and otherworldly creatures hunting the few remaining survivors of the attack. also, the nuclear cooling system of europa-1 has been damaged by small arms fire, and core breach caused by overheating will destroy the starship in a matter of hours.