The Last Stand (Star Trek: The Next Generation, No. 37)

In the middle of a routine mapping mission, Captain Picard and the crew of the "U.S.S. Enterprise" TM encounter a culture just on the edge of developing warp drive technology. When they survey the planet, they are startled by the sudden approach of thousands of spacecraft from an aggressive alien race bent on destroying this emerging culture. Picard has only days to resolve a conflict that has been going on for millennia. If he fails, billions will die, yet if he succeeds, he will unleash a powerful new threat to the Federation.