System Shock 2 follows many conventions of a role-playing game, with considerable first-person action involved.  Like "Thief" (whose engine was used to create "System Shock 2"), the objective is not  necessarily to kill everything in sight, but to achieve the required goals.  You play a new recruit onboard a starship fighting against alien viruses  and the ship's evil main computer.  You start with some initial skills of  your choice and are given chances to improve your skills as the games  progresses.  The goals you need to achieve range from mundane to  complicated: locating crucial items, unlocking rooms, restore power to  equipments, hacking into terminals, scouring dead astronauts' journals for  clues, collecting chemicals from laboratories that enable you to research  on technologies that help you combat your enemies, find tools that repair  and maintain your weapons, and so on.  My favorite puzzle is finding pieces  of a secret code hidden among the numerous video screens on one of the  levels.