The X-Files - Season 9 Episode 16 William The truth is out there.
Overview: Doggett catches someone in the X-files office trying to steal documents pertaining to Mulder's sister. The intruder is a severely scarred man who blames his disfigurement on Government conspirators. He claims the source of that information - and, in fact, the one who helped him gain entry - is Mulder. Although he refuses to divulge the missing agent's whereabouts, the mystery man has enough knowledge of the X-files and its history to lead Doggett to suspect that he just might be Mulder himself. Scully's fears for the safety of her son are elevated when he is injected with an unknown substance. With the mystery surrounding William becoming even darker, Scully must allow herself to send him away for his own safety.
Comment
I can't believe I'm the only idiot writing an X-Files review here. Anyway, I still don't understand Piper Maru, it was a good episode but not really worth the praise it gets, and let's be honest, the Monster of the Week episodes were always better than the Mytharc ones... ... mostly better. There was were some pretty darn good mythology episodes, and there were ones like Piper Maru that were OK but not great. It was probably the ultimate television drama and an influence to everyone that wanted to achieve greatness on the small screen. It really showed what you could do with a one hour time slot and we were all blessed to have this done, not only because it was done so well, acted so well, and written in a way that invested the audience, but because it pathed the way for people that wanted to achieve that level of greatness. Television got better because of the X-Files, movies got better... and then politics came and it all sucks now, but for a little while, the X-Files raised the bar for everything.