Season 5 has a lot of "conspiracy" episodes and "spy" plots. I found myself missing the good old episodes involving investigations of strange creatures. I was missing the horror. And here we have this episode - Folie a Deux - a pure horror episode! The story runs very smooth, no rushing and no boring moments. There are some nice chilling shots (with a reference to the good old zombie movies), nice cinematography and well-paced story. The plot (although nothing original) could sustain even a full-length movie, I believe. It is very interesting also how Moulder approaches the case - skeptical at the beginning, hesitant after some clues and finally - near psychotic and put into a hospital due to his absurd statements. To me it was one of the best episodes from Season 5.