. S . S . ▁Sul aco ▁as ▁well ▁as ▁L V - 4 2 6 ▁and ▁Ph ob os ▁in ▁order ▁to ▁investigate ▁the ▁Union ▁of ▁Progress ive ▁Pe op les ▁( UP P ) ▁and ▁W ey land - Y ut ani ' s ▁invol vement ▁in ▁X en omorph ▁development . ▁The ▁game ▁incorpor ates ▁a ▁per made ath ▁mechan ic . ▁If ▁a ▁marine ▁falls ▁in ▁battle , ▁another ▁in ▁the ▁party ▁will ▁take ▁their ▁place . ▁The ▁player ▁los es ▁if ▁the ▁entire ▁party ▁is ▁w ip ed ▁out . ▁F allen ▁characters ▁can ▁be ▁replaced ▁with ▁any ▁of ▁fifteen ▁disp la ced ▁mar ines ▁found ▁throughout ▁the ▁game , ▁each ▁with ▁their ▁own ▁unique ▁dialog . ▁ ▁Plot ▁The ▁USS ▁Se ph ora ▁has ▁discovered ▁the ▁USS ▁Sul aco ▁ad rift ▁in ▁space ▁after ▁the ▁events ▁of ▁Ali ens ▁and ▁Ali en ▁ 3 . ▁The ▁Col onial ▁Mar ines ▁are ▁sent ▁ab o ard ▁the ▁Sul aco ▁to ▁investigate ▁and ▁recover ▁the ▁life ▁form ▁detected ▁ab o ard ▁the ▁ship . ▁ ▁Development ▁ ▁Ali ens : ▁Inf est ation ▁was ▁developed ▁by ▁Way For ward ▁Techn ologies ▁and ▁G ear box ▁Software ▁and ▁was ▁originally ▁intended ▁to ▁be ▁a ▁tie - in ▁to ▁the ▁PC ▁and ▁console ▁Ali ens : ▁Col onial ▁Mar ines ▁game . ▁When ▁doing ▁their ▁research , ▁Game ▁Director , ▁Adam ▁Tier ney , ▁focused ▁heavily ▁on ▁R id ley ▁Scott ' s ▁Ali en ▁and ▁James ▁Camer on ' s