At a rendezvous on an uninhabited planet, Captain Keel and two other Starfleet officers tell Picard that they suspect a growing conspiracy in the upper ranks of Starfleet.

Alarmed by Keel's accusations, Picard orders Lieutenant Commander Data to review all Starfleet directives during the past six months. Picard's concern escalates when Keel's vessel mysteriously explodes, killing all aboard, and Data's research reveals abnormalities in the highest levels of the Starfleet command.

Convinced the Federation's security is at risk, Picard steers the Enterprise toward Earth to confront Starfleet's top admirals. After requesting a meeting with the officers, Picard and Riker are invited to dinner to discuss the problem.

Prior to dinner, Admiral Quinn, who several months earlier had warned Picard that subversive elements had invaded Starfleet, visits the U.S.S. Enterprise. Suspicious of the admiral, Picard orders Riker to watch him closely before joining him on Earth. Picard's instincts turn out to be correct: as soon as Picard beams down to join the other admirals, Quinn attacks Riker, rendering him unconscious.

After Lt. Worf and Dr. Crusher join forces to subdue the incredibly powerful Admiral Quinn, they discover that a parasitic being has invaded Quinn's body and it is controlling all of his brain functions. When Riker regains consciousness, he beams down to Earth, pretending to be one of the parasitic-controlled beings.

At the dinner, Riker realizes that the Starfleet command is controlled by the parasites. Fortunately, he and Picard are able to kill the parasitic-infested admirals, as well as the species' mother creature. However, Data later discovers that the mother creature sent a homing beacon to an unexplored region of our galaxy before being destroyed ...