The Lone Gunmen - Season 1 Episode 12 -

In this episode, we learn some secrets about Yves, and according to Frank Spotnitz, this is where we start to see the Gunmen's mythology open up.Morris Fletcher is apparently snatched by aliens (apparently the Gunmen in disguise!), and Fox Mulder runs into Jimmy Bond. I've heard rumors that this episode may also feature a second appearance by Skinner, but I haven't seen any concrete evidence for it.