So, I have an idea for a mini campaign - something like 3 to 6 sessions max. Fallout setting because I can't figure out any other setting that would work with this set up - I don't want fantasy, I don't want super high tech sci-fi and I don't want real history because of the research requirements
Everyone lives as a normal person in a city built over a vault - life is tolerable but the government is very corrupt. People who speak out against it are exiled, tortured, or shot. Still better than living in the Wastes.
A new school opens up that teaches technological skills. Your characters are the most adept students there. You earn the trust of the teachers there and they tell you that they are part of the Brotherhood of Steel. They want to liberate the city from the government and need your help. They want you to work as their undercover agents in the city, gathering information and building support for them. When they roll in, they want a smooth take over, not a civil war. That's a lot easier if they have leverage over the city's power brokers.
Emphasis on the game is actual spywork - i.e. building a network of assets, getting blackmail/secrets on important people and not getting caught. If you get caught before the Brotherhood rolls in, your character will almost certainly die. Combat will be super brutal and should be not seen as a good idea.
I see 2 options with system and feeling for the game:
Night's Black Agents with Dust, Burn, and Mirror options enabled - you're a skilled person, not Bourne or Bond. Think more like post-apocalyptic Tinker Tailor Soldier Spy.
Hillfolk - emphasis is more on character development and morality - more like post-apocalyptic Game of Thrones
The main thing for me is the actual spywork procedural aspect and morality - I already did the tactical/commando/investigative NBA game with Tribes - I want it be more about the morality and interpersonal stuff.