I read alot about the plot in the FFG news but I wonder if the theme is really an inherent part of the game or more "glued on". Well I know it's not "glued on" but does all the plots and everything really contribute to the gameplay and the general feel?

Amitloaf said:

Yes. Accomplishing goals in your plotlines contributes directly towards earning Victory Points. Depending on how your plots resolve (at the end of each game week), you can receive up to 10 VP (or lose that much). Plots also provide side benefits and complications, sometimes in concert with earning VPs, that last for the remainder of the game.

In addition, all of the Light and Dark Twilight cards apply to one of an investigator's plots. Playing them while the relevant plot is "active" allows you to play more powerful cards in an easier fashion.

The idea that Kevin and Dan are trying to achieve is that the game is more of a detective-noir narrative, where the murder (and the conspiracy behind it) are what tie all of the investigators together. Each is trying to earn/restore favor with the NAPD, to further their own agendas. Whether those agendas are noble/selfish/good/evil/practical/foolish is up to each player to determine (and then take appropriate action on).