An anthropological research station overlooking a primitive society called the Mintakans suffers a catastrophic malfunction that allows the holographically shrouded station to become visible to its Mintakan subjects. A Mintakan named Liko (Ray Wise) witnesses the Enterprise's ensuing rescue mission and is critically injured in a fall. Rather than letting him die, Crusher beams him aboard, repairs his injuries, and erases his short-term memory. The memory wipe doesn't take, and Liko tells the story of what he witnessed aboard the Enterprise to his fellow Mintakans. He believes "the Picard" is a god who gave him back his life.
As an episode that dramatizes the purpose of the Prime Directive and the dangers of cultural contamination, "Who Watches the Watchers" is perhaps definitive. The question of whether you can study something without running the risk of affecting the results is answered here by a series of accidents that ultimately suggests an entire religion could eventually be formed around "the Picard" as based on Liko's experience.
But this episode is also definitive as an example of short-changing a concept by way of extreme microcosm. An entire planet's culture (and this has frequently been one of my complaints about Trek) is represented based solely on a dozen villagers who seem more like isolated nomads than part of a real, larger society. Meanwhile, characters in this story make sweeping assumptions that are almost absurd in their broadness. The idea that Liko's experience will "inevitably" lead to a religion worshipping Picard strikes me as an unlikely conclusion given what we know about the Mintakans. Surely there must be other societal factors in play in order for a religion to take hold and flourish. One man speaking secondhand nonsense cannot change the world.
For that matter, this episode's take on religion seems awfully simplistic. While it would be against the Prime Directive to allow Picard to be seen as a god, Picard has a speech here that seems to be against religion at all. The Mintakans left behind their supernatural beliefs generations ago, and Picard sees that as an achievement from the "dark ages" which he does not intend to allow they return to. Of course, there's no mention of the status of human religion. (I suppose the 20th century was still the "dark ages" because of all the silly human religious beliefs that persisted?)
In the latter acts, Picard tries to convince Nuria (Kathryn Leigh Scott) that he is not a god but simply part of a society that has more knowledge. This concept seems to arise from a what-if premise: What if you could show a person from 2,000 years ago what the world looks like today? The story does its best to create a sense of wonder in this, but never quite reaches takeoff velocity.