Spoilers...If you care.
I thought it was Vault 1. Which was specially made for scientists which were trying to find a cure for damaged tissue (Your character gets his arm cut off by Attis, and then thrown down into a vault area, and then the vault citizens use their technology and fix you up good.) So then Attis takes his mutant army down there to try to make mutants sterile once again (and the whole 3rd act, your lead to believe that they were after some weapon). Yup thats pretty much it.
Secret Characters are (as stated b4):
Patty: Vault Security officer. She gets killed at the end of the game. I really don't know why the hell a secret character that actually appears in the game and dies would be a secret character.
Rhombus: Rhombus is about 60 or 70. He's an geezer, and isn't to fond of ghouls. He also dies in the game, so another secret character that kinda pisses me off. He reminds me of Solidus Snake. He's even got the eyepatch down.
Vault Dweller: This character also appears in the game and is the final one you get. I think it is cool that he makes an appearence, but I wouldn't go so far to make him playable. If you try to make the game in between Fallout 1 and 2, try not to fuck up the timeline by making the vault dweller go way outta his way to take out Attis and the mutants....again. He was cool to see, but the reminising on Fallout 1 kinda pissed me off, he did it way to much, kinda making sure I got the point.
All the secret characters were kinda thrown in at the end of the game, but having characters that appear in the game as opposed to no characters is better then nothing I guess.
They did have good voice actors. Earl Boen (Terminator 2 star, Dr Silberman), Michael Bell (Raziel), Tony Jay (Elder God), and of course Cam Clarke (Appears in almost every game, Pukk in Beserk and Liquid Snake in Metal Gear Solid 1 and 2) all are incredably good. The dialoge does dound bad, but thats because it's chopped up into fragments so we can ask retarded little questions that don't make a difference anyway. Why not just set up a scene and make it dramatic, like how the whole Legacy of Kain series or Metal Gear Solid does it. That way at least the dialoge will sound more realistic.
The game isn't bad though. I would give it a 7/10 or maybe and 8. But lets get one thing straight here. This game is nowhere as good as Fallout 3 would have been. Cancelling FO3 for FOBOS2 is perhaps the most retarded thing ever. This would have been a good filing game to keep the lesser die-hard fallout fans occupied until FO3 came out, but now that ain't gonna happen, so that kinda sucks.
I guess I will give my full review (and reviews on all the games) later.
Oh Ya. No Mutants Allowed and Duck and Cover get thanked in the credits. It says thanks for the laughs duckandcover.net and nma-fallout.com.