"Conspiracy" is the 25th episode of the syndicated American science fiction television series Star Trek: The Next Generation, originally aired on May 9, 1988, in the United States. The premise was conceived by the show's creator Gene Roddenberry in a single sentence overview titled "The Assassins", being expanded into a thirty-page story by Robert Sabaroff. From this, the teleplay was produced by Tracy Tormé and the episode directed by Cliff Bole.

Set in the 24th century, the series follows the adventures of the crew of the Starfleet starship USS Enterprise-D. Following a meeting with a fellow Captain, the strange behavior of high-ranking officers leads Jean-Luc Picard (Patrick Stewart) and the crew of the Enterprise to uncover a conspiracy of senior Starfleet officers possessed by parasitical aliens who are preparing to invade the United Federation of Planets.

Numerous make-up effects were created by make-up supervisor Michael Westmore, including an exploding head using raw meat. There were concerns by producers that some of the effects were too graphic, but after a viewing by a staff member's son, they decided to broadcast it uncut. The episode has subsequently been included in some best episode lists of the series, and won one of three Primetime Emmy Awards for The Next Generation's first season. Controversial when originally broadcast, it was initially banned from broadcast in the United Kingdom, and required a warning before airing in Canada.

While the Enterprise is en route to Pacifica on a scientific mission, Captain Picard (Patrick Stewart) receives a highly confidential message from an old friend, Captain Walker Keel (Jonathan Farwell) of the USS Horatio. Keel refuses to discuss his concerns, even though the frequency he is using is supposed to be a secure channel, and insists on a face-to-face meeting. Once they arrive at Dytallix B for this secret meeting, the crew discover the Horatio and two other Federation ships already present. Picard beams alone to the surface and is met by Captain Keel and the other captains. Keel reports strange orders from Starfleet headquarters and what he implies are suspicious deaths of Starfleet officers, and expresses concern of a conspiracy. Picard refuses to accept this without proof, but Keel warns him to remain wary. When the captain returns to the Enterprise, he is challenged by Counselor Troi (Marina Sirtis) about keeping his contact with Keel secret. Picard is skeptical about the conspiracy, but says that he trusts his old friend completely. The Enterprise resumes its previous journey. Picard has Commander Data (Brent Spiner) review all orders issued by Starfleet Command in the past six months. While the ship is still on its way to Pacifica, a disturbance is detected in nearby space. Upon investigating this, the Enterprise discovers a mass of debris that surely must be the shattered remains of the Horatio. Data completes his study and finds several strange orders from the senior levels of Starfleet. Picard informs his senior staff of the conspiracy theory, and orders the Enterprise to Earth.

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