In 2364 a species of neural parasites secretly took control of senior Starfleet officers until Picard and Riker destroyed the creature leading them.
In 2364 Captain Walker Keel warned Picard of a conspiracy within Starfleet, and his ship was destroyed soon afterward. Picard took the Enterprise-D to Earth and found that several admirals and officers at Starfleet Headquarters were behaving strangely. Small parasitic creatures had entered their bodies through the neck and controlled their minds.
The parasites were directed by a larger mother creature living inside Lieutenant Commander Dexter Remmick. Picard and Riker destroyed Remmick and the mother creature with phasers, killing the other parasites. Data found that Remmick had been sending a homing signal into deep space, suggesting the species meant to bring more of its kind to Earth.