Wow, that really was a bad movie. Apparently, the plot of Plan 9 from outer space is that aliens want to destroy Earth because they're worried humans will invent a 'solanium' bomb that will detonate sunlight, and thus destroy the universe. So they spend a week buzzing around one particular cemetary and reanimate three corpses. Really. The three aren't representative of their alien hordes, they go before their supreme leader and proudly announce they have THREE. Then they destroy one to sort of freak some humans out (that's actually their plan), and when the humans head to the cemetary they decide to invite them into their ship and monologue at them and tell them in excruciating detail about the bomb they're afraid that humans will build, which the humans themselves had never even considered prior to this. Then the humans punch the aliens until their ship catches fire and explodes.
It probably didn't need the riffing to be funny, but the riffing was pretty damn funny. >:)