Srsly, with ray guns, atomic bombs, robots, flying saucers, giant spiders, paranoia, rockabilly, and all that great stuff.
Oh, and the party would be made up of average joes in a situation way over their head. Like, a government bureaucrat, awol soldier, nurse, and the dude that pumps your gas.
There is one called GURPS Atomic Horror. And there is coming up The Menace From Beyond