Picard uncovers a plot inside Starfleet Command.
Picard is called to a secret meeting with Captain Walker Keel and two other captains, who suspect that something is badly wrong in Starfleet Command. Soon afterward Keel's ship is destroyed, and Picard takes the Enterprise to Earth to investigate.
At Starfleet Headquarters Picard and Riker discover that parasitic alien creatures have taken over senior officers, including Admiral Quinn, who had hinted at the trouble earlier in the season. Riker pretends to have been taken over in order to get close to the leaders.
Picard and Riker destroy the mother creature inside Lieutenant Commander Remmick, killing the parasites. Data finds that a homing signal was sent into deep space, hinting that the threat may return. The gory climax was unusually graphic for the series.