Under Starfleet transmission code 47 — the utmost secrecy and urgency — Picard is called to the surface of a desolate, abandoned mining planet by his good friend, Captain Walker Keel (Jonathan Farwell) of the USS Horatio. Keel warns Picard of a conspiracy growing within Starfleet Command, and ominously tells him to trust no one. The Horatio is shortly afterward destroyed in a disastrous implosion. Coincidence? Yeah, right.
"Conspiracy" is the tensest and most unpredictable of TNG's first season, starting with Keel's wide-eyed "No!" when Picard asks if their meeting can be done with less cloak-and-dagger sneakiness, and proceeding through the slowly building realization that an alien threat has come from beyond and is now attacking by manipulating from within. Ironic that one of the most involving of TNG's first season is also essentially an anti-Trek storyline, in which a malevolent alien threat must be exposed and destroyed with brute force (not to mention visceral reactions of unmasked disgust, particularly from Picard), and an ominous ending that is not at all reassuring.
Admiral Quinn, who alluded to the conspiracy in "Coming of Age," turns out to be under the alien influence here, and throws around like rag dolls (in the following order) Riker, Geordi, and Worf, before being put down by a phaser blast from Dr. Crusher, who subsequently discovers the parasitic being attached to Quinn's upper spine. Meanwhile, Picard's meeting with the admirals at Starfleet Headquarters on Earth is a nicely played escalation of subdued horror — with idle conversation that then proceeds to a meal of live worms, and finally the revelation that Riker has been compromised — or has he?
The mother of all the aliens is hiding in Lt. Cmdr. Remmick (which is somehow amusingly appropriate), which makes for a rather creepy sequence: Somehow, when he says the aliens seek "peaceful coexistence" I'm less than convinced. Neither is Picard. "Conspiracy" is a brawny hour of X-Files-style Trek, and might be the only episode of Trek where a man's head so awesomely gets blowed up real good. The episode perfectly sets itself up for a sequel that, alas, would never come.