I remember this was one of the first episodes of the X-Files that I ever watched (back when it first aired in '99) and before I re-watched it after buying the DVDs all I could remember was the ending (the agents repeatedly not knowing what was real and what was just an illusion).
Mulder and Scully are basically eaten alive by a huge plant inside a mountain that imprisons them with a hallucinogenic "slime" that confuses them as to what is real and what is simply put in their heads by the drug.
Still, to this day, I think this is one of the most interesting and riveting episodes of the entire series. I mean it really makes you think, what is reality? Is it limited to what we physically experience or rather, what our mind tells us that we are experiencing?