The Big list of RPG Plots
developing your own.
A nice PDF of the list is here:
One of the ones I particullary like is:
No One Has Soiled The Bridge
The PCs are assigned to guard a single vital spot (anything from a mountain pass to a solar system) from impending or possible attack. They must plan their defensive strategy, set up watches, set traps, and so on, and then deal with the enemy when it arrives.
Common Twists & Themes: The intelligence the PCs was given turns out to be faulty, but acting on the new information could result in greater danger - but so could not acting on it, and the PCs must choose or create a compromise. The PCs learn that the enemy has good and sympathetic reason for wanting to destroy the protected spot.