Possession of key crew members leads to a hostage crisis.
Answering what seems to be an old distress signal from a moon, Troi, Data and O'Brien are taken over by disembodied entities after a shuttle crash. The beings first claim to be the lost crew of the USS Essex, a starship that disappeared two centuries earlier.
The possessed officers seize Ten Forward and hold crew members hostage, including Keiko O'Brien and her baby, and Picard trades places with some of them. He soon realises the entities are not the Essex crew but criminals who had been exiled to a penal colony on the moon. The crew traps the beings and forces them to leave their hosts by threatening to expose them to space, and the officers return to normal.