The players are a six-man crew of specialized soliders at a remote Alaskan nuclear silo during the 1960s, responsible for the care, maintenance, and launching of nuclear weapons.
Day one, they get a basic tour of the small base. Silo with two big missiles, launch control, mess hall, personal quarters. Maybe introduce a handful of well-fleshed out NPCs. The doctor, the cook, the two big guys with guns, the commander. Feel free to make the supporting cast larger or smaller as you see fit, but try to keep it small.
Day two, the base receives a cryptic encoded message, broadcast directionally towards their base. Triangulating the coordinates, they realize it was tightly broadcast in a straight line to only their base. Signs point that it came directly from over the Beiring Strait. Russia.
That night, one of the NPCs crack. Maybe the cook tries to poison the food, maybe one of the guards tries to murder everyone. This maniac is overwhelmed. Under duress, he admits he is a Russian traitor, mentions something about purging the capitalist plague in a storm of hellfire and flame, that his plan of getting the launch codes fell through. With a smile, he mentions he has a partner and they are as good as dead. Before the PCs can pump him for more information, he swallows a cyanide pill hidden in a false tooth and dies violently.