In a nutshell, IE is a mission based GZDOOM TC for DOOM II. Take doom, mix in a dose of unreal and turok, add a splash of metroid and maybe a little bit of system shock 2 and you'll not be far from the target. You play as an officer of the Interstellar Enforcement Patrol, tasked with policing the obscure fringes of humanities bustling space empire in the future of the DOOM II timeline. You generally do this by shooting things.