TNG Season 3 Episode 4: Who Watches The Watchers

Foreshadowing Star Trek: Insurrection, a Federation research team has been revealed to the indigenous lifeforms on a planet with a (very) pre-warp civilization, causing a breach of the Prime Directive, necessitating Commander Riker and Counsellor Troi to go undercover as the Vulcan-eque Mintakans to rescue a researcher. Hilarity ensues. That is, if you find Picard being referred to as a God and having people sacrificed to him hilarious.
This one’s such a good foundation for new viewers as to quite why the Prime Directive is so important in the Star Trek universe that it was drawn upon for the opening sequence of Star Trek Into Darkness. It shows Next Generation at its thoughtful best.

Plot

The Federation starship Enterprise, under the command of Captain Jean-Luc Picard, arrives at the planet Mintaka III to resupply and repair a Federation outpost being used to monitor the Mintakan people, a proto-Vulcan race near a Bronze Age level of cultural development. The outpost is built into the side of a cliff and camouflaged by a holographic projection of a rockface.
As the Enterprise provides assistance, the projector malfunctions and the outpost becomes fully visible. Liko, a Mintakan, sees it and attempts to approach, but suffers an electrical shock that causes him to fall off the cliff and sustain critical injuries. When Chief Medical Officer Dr. Crusher rushes to provide aid, she realizes the injuries are too severe to treat at the scene and has him transported to the Enterprise for treatment despite the action violating the Prime Directive. Liko becomes conscious and witnesses everything occurring around him, and focuses on Picard giving instructions. Dr. Crusher is able to heal Liko and attempts to wipe his memory of the incident before returning him to the planet. First Officer Riker suggests that he and Counselor Troi disguise themselves as Mintakans in order to search for Palmer, a missing member of the anthropological team, and to monitor Liko, to make sure the memory wipe worked. They discover to their horror that it did not, as Liko recalls an image of "the Picard", and has convinced other Mintakans that the Picard must be their god.
Troi and Riker subtly try to dispel the myth of the Picard, which gains traction until a hunting party arrives with a delirious Palmer in tow. While Troi provides a diversion, telling the clan that another "like Palmer" is heading for the caves, Riker ties up an elderly man who was left behind to keep an eye on Palmer; he and Palmer flee the area and escape back to the Enterprise. The Mintakans capture Troi and consider killing her to mollify the Picard, leaving Picard to take steps to rectify the situation without further violation of the Prime Directive. He transports Nuria, the leader of the village where Troi is being held, to the Enterprise and attempts to show her that he and the rest of the crew are mortal, including having her witness the death of a crewman in Sickbay.
Picard returns with Nuria to the surface in the middle of a thunderstorm, which Liko has taken as a sign of the Picard's anger. Nuria attempts to reason with Liko, but he demands first-hand proof of Picard's mortality and aims at him with a bow and arrow. Picard insists that Liko should shoot if the only evidence he will accept is Picard's death. Liko fires, but his daughter shoves him so that he only wounds Picard; seeing him fall and bleed, Liko and the others come to accept that Picard is not a god. Picard and Troi return to the Enterprise, and after he is treated, Picard returns to the surface one last time, and explains to the Mintakans that the Federation will remove the outpost and allow them to develop on their own. Before Picard leaves, Nuria gives him a Mintakan tapestry as a gift.