Macdad, if you don't have the Plasma cannon or BFG then your playing the Shareware demo, which only included the first episode (knee deep in the dead) and not the following episodes (The shores of *censored*,Inferno,Thy Flesh Consumed).
On the topic of Doom 3, I thought it was kind of silly, almost predictable.. I'd be going down a corridor..
Me:well, I haven't seen an enemy in a while, probably one around here some...
Zombie: Roar!
Me: oh, hi there. have some lead.
Zombie: Darn.
Me: I wonder when I'll see another Imp?
Imp: You Raaaang?
Me: Oh, hi there. have some lead.
Imp: Please Sir, can I have some more?
Me: sure!
Imp: oh dear I am dead.
Cacodemon: you killed george! How dare you!
Me: you mean the zombie or the Imp?
Cacodemon: who cares! Now you die
Me: No.
cacodemon: Blast! you have defeated me!
lol sorry about that I got a bit carried away. It's an ok game though, my comp is to old to play it well though.