On stardate 41775.5, the USS Enterprise is en route to Pacifica when Captain Picard receives a priority transmission from Captain Walker Keel (Jonathan Farwell) of the USS Horatio. Keel tells him they need to talk face-to-face on the planet Dytallix B in the Mira system as soon as possible â and not to trust anyone. The Enterprise arrives at the desolate planet where Worf detects three other Federation starships â the USS Renegade, (commanded by Tryla Scott (Ursaline Bryant)); the USS Thomas Paine, (commanded by Captain Rixx (Michael Berryman)); and the USS Horatio. None of the ships offer greetings and Picard beams to the planet alone.
Picard materializes before a mine entrance where two figures level phasers at him. Captain Keel steps from the shadows and barrages Picard with a series of personal questions to be sure Picard is not an imposter. Keel then introduces Rixx and Scott and gets down to business. The three declare there have been strange patterns emerging within Starfleet â unusual orders, irrational proposals, unexplained deaths and accidents, and together they believe a new threat is emerging within their ranks. Picard is incredulous without proof, but Keel insists those at the top level of command are changing somehow. Keel insists that Picard keep his eyes open and to communicate with him covertly.
Continuing course to Pacifica, Picard has Data review all orders Starfleet has given in the past six months to find any "anomalies". Next, the Enterprise comes upon a debris field which Worf determines to be the wreckage of the USS Horatio. Picard fills Riker in about the secret meeting and explains that during their visit to Relva VII, Admiral Quinn had indicated there was a subversion brewing within Starfleet. He believes Keel was warning of the same thing and destruction of the Horatio must have been sabotage. Data enters and relays his findings of a number of abnormal directives given with subtlety at the highest ranks. He suggests it could be an attempt to control vital areas of Federation space. Riker wonders if it could be a prelude to invasion. Picard boldly decides to go to Earth and find out directly from the top.
The Enterprise heads at maximum warp to Earth and upon approach, is greeted by three senior officers â the Vulcan Admiral Savar (Henry Darrow), Admiral Arron (Ray Reinhardt), and the familiar Admiral Quinn (Ward Costello). In the background is Lt. Commander Dexter Remmick (Robert Schenkkan). Savar inquires about the unscheduled visit and Picard requests a meeting. Savar tells Picard and Riker to join them for dinner in 20 minutes. Quinn mentions he'd like to pay a visit to the Enterprise instead and beams aboard. To Picard's discouragement, Quinn says he doesn't recall the matter he discussed on Relva and states Picard must have misunderstood their discussion of Starfleet's problems with integration of new alien races. Picard excuses himself for the dinner engagement, but as Quinn heads off, Picard takes Riker aside to tell him Quinn is an imposter. Riker is skeptical, but Picard tells him to keep an eye on him.
After Picard departs, Riker enters Quinn's quarters and finds the man staring into a metal case. Quinn explains it's a new life form discovered during a survey mission and reveals a squirming, bug-like creature with a nasty set of pincers. Quinn claims it to be "a superior form of life" leaving Riker puzzled. Riker suggest letting a science officer see it, but Quinn coldly replies, "It won't like your science officer. It does like you." With surprisingly superior strength, Quinn grabs Riker's arm. When Riker tries to pull away, Quinn attacks him, and throws him across the room when Riker calls for help.
Meanwhile on Earth, Picard tries to explain his visit to the Admirals. When Admiral Arron offers a toast to the Horatio, Picard asks about the cause of the ship's destruction. Savar states it was negligence on the part of her captain.
Back on the Enterprise, Worf and Geordi run to Quinn's room and find Riker on the floor. Quinn calmly tries to leave, but La Forge blocks the door. Quinn takes a swing at Geordi who fires his phaser, but the Admiral absorbs the beam and quickly recovers. Quinn then throws La Forge through the doors and out into the corridor. Worf takes a defensive stance and strikes Quinn, who isn't fazed. He tries to throw Worf as well, but Dr. Crusher enters and hits Quinn with multiple phaser blasts to floor him.
While examining Admiral Quinn in sickbay, Dr. Crusher notices a thornlike protrusion on the back of his neck which squirms when disturbed. Alarmed, she begins a cranial scan. Picard contacts the Enterprise for an update and Dr. Crusher informs him of the events involving Admiral Quinn. She explains that he has some kind of parasite attached to his brain, explaining the odd behavior and incredible strength. They cannot be removed without killing the host. She tells him other infected people will have a gill protrusion on the back of the neck and that phaser stun has little effect against them.
Picard arrives for dinner only to discover squirming larvae being served as the main course. The others gulp them down, and Picard tries to escape but finds Riker blocking his path. Admiral Arron checks Riker's neck which is revealed to have the parasite gill protruding from it.
Picard demands to know who and what they are. Savar states they have come a long way to join humanity, serving as the brains while the human body will be the brawn. He indicates they will soon control everything, especially now that they have the Enterprise.
Picard watches Riker grab a handful of maggots. While the others grin with sinister delight, Riker prepares to put the squirming larvae in his mouth, but he suddenly draws his phaser and blasts a guard. Tryla draws her weapon, but Riker cuts her down. Picard acquires the fallen guard's weapon and together he and Riker take down the remaining officers. As Arron slumps to the floor a parasite crawls from his mouth and scuttles under a nearby door.
The two follow the parasite where it runs to Remmick sitting at a computer console. Remmick turns around to face them and allows the bug to crawl up his leg and enter his open mouth. Riker and Picard are horrified as Remmick's neck bulges from the movement of other bugs. He says, "We mean you no harm. We seek peaceful co-existence."
With their phasers at maximum setting, Picard and Riker simultaneously fire at Remmick, whose head and upper torso explode. Dozens of dead alien bugs fall from the corpse, then a large creature rises from the remains of Remmick's lower torso and shrieks in anger. Riker and Picard fire until it vaporizes.
Picard and Riker return to the ship. Dr. Crusher reports the parasite inside Quinn has dwindled to nothingness. Apparently, they cannot survive without the mother creature that was inside Remmick. The lingering question is what those creatures are and where they come from. Data indicates that before Remmick was killed, he was sending a signal toward a distant quadrant of the galaxy. It appears to be a homing beacon, perhaps to guide something to Earth.