I wanted to put this under the 'Unofficial Mythology' category but there is a key scene deep in this episode where Mulder pretty much refutes the fact that the current case is involved with anything from the mythology (the scene where Scully says Mulder is not being objective because of his sister and Mulder, quite convincingly, says his sister has nothing to do with it). During that particular scene, Mulder is pretty adamant that his objectives and beliefs are not strictly run by his emotions towards his sister and isolated events from the past.

And this episode is one of those bait and switch episodes where you think, maybe, Mulder is acting extra sensitive because this all has something to do with his sister but, in the end, Mulder just happens to have a heart and it is extra sensitive to child kidnappings. . .and it doesn't hurt that there is some mental voodoo going on too. I found a lot of similarities, oddly, with the first season episode '
Young at Heart' in which Mulder kind of went against his normal routine and fought hard for a case, emotional state be damned.
One of the most telling character moments between Mulder and Scully occurs in this episode and it is pretty powerful especially considering it is one of the rare times the two characters share a NEGATIVE moment. When Mulder tries to resuscitate Kaylee from Firefly and fails, Scully tries to get him to stop and Mulder, angry and emotional, pushes Scully away. Scully looks shocked for a moment and so does the audience. In that moment in a run of the mill 'MOW' we witnessed a new wrinkle in the Mulder-Scully relationship. Most of the time we see the two having 'moments' that makes them connect and grow close. . .in this case, Scully sees a line even she can't cross. It was nice to see this. . .even if it was off putting and sad.
In regards to the plot, I like that The X-Files can throw things like aliens, ghosts, chupacabras, etc at you but the scariest episodes can be the episodes involving the deepest, darkest holes of human nature. You can show me all the Tooms you want but seeing a grown man breath heavily over a little girls bed gives me the willies.

And the actress they cast to be the mental conduit between Kaylee from
Firefly and herself was so wonderfully odd and unique looking that her appearance alone increases the success of the episode. It also doesn't hurt that David Duchovny is on top of his game here, being believable and heart breaking.
The episode doesn't exactly end well and it isn't quite as well known as other efforts in the season or the show's run, bu 'Oubliette' has enough scares, surprises, and character moments to make it a very important watch.