This is a really interesting trilogy. the first Doom is a great shooter, with a ton of secret areas and levels, and  that classic doom game play. The second one picks up right where the first left off in terms of game play. The third, at this point (i mean who hasn't played the older dooms on 10 different platforms by now) is the most interesting game. It begins with a narrative segment, going through the mars base and talking to all of the grumpy marines, introduces you to the sentry bots, etc. Then the game pulls a "jk" and goes right back to the roots of old doom. the farther in you get, the more then game starts to throw back. It is really interesting to see how id re-imagined the UAC mars base in the early 2000's. All of the old enemies and items return, and its really great to see how they have been transformed into a more modern design(though doom III is starting to feel more classic at this point.). Floors covered in poison and magma, what are essentially key cards, and of course, the whole hell on mars thing. This franchise is a great piece of video game history, and a "real" game at heart. i always like how carmack never focused on story, but instead on mechanics, and cutting edge tech. it is pure uninterrupted shooter gold.