The X-Files - Season 9 Episode 16 William

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.