Possession (Star Trek: The Next Generation, No. 40)

Eighty years ago, bodiless entities brought a plague of violence and bloodshed to the planet Vulcan. The nightmare ended only when the entities were trapped inside special containers.

Now, on the eve of a galaxy-scale scientific exposition, the containers have been opened, freeing the malevolent entities to possess the minds and bodies of all they encounter, including the crew of the Starship Enterprise?. Friends turn into foes, and no one can be trusted as Captain Picard faces a deadly and insidious threat. Unless the entities can be stopped once more, they will spread their madness throughout the entire federation.

Eighty years ago, bodiless alien entities brought a plague of violence and bloodshed to the planet Vulcan. The terror was ended only when the entities were trapped in special containers and walled away behind forcefields. But now, some greedy Ferengi have opened the containers, releasing the rapacious entities to possess the bodies of the EnterpriseTM crew. No one can be trusted -- anyone could be host to one of the alien things -- and Captain Picard must devise a way to defeat the entities before they spread throughout the Federation.