This one has been my favourite episode of the whole series since the first time I watched it, and rewatching it after 4 years confirmed it.
Although in a serious battle with the last episode (Come Undone), and even if my love is practically the same for both episodes, this wins for 2 reasons: even if the final part of the episode is not as good and creative as the rest, the whole concept is pretty creepy for a kids show, and do it is the episode. Second reason: it's original. If you ever saw something with the same/similar plot of this episode, please tell me its title.
If I have to search some bad things in this episode, probably I would say that when the mystery is revealed, the whole explanation given by the two "bad guys" at the end is too long and reveals all the details in a really schematic way, and - most important - it's unrealistic. If all the episodes' evil characters/monsters (if not all most of them) had a pretty solid an realistic explanation/way in which they done whatever they done, this one is too unrealistic and - if I can say this - surreal.
But for a kids show, it's ok, I mean if a thing like this would happen in a more "mature" tv series THAT would be bad.
The reason why I give 9/10 to this it's because - for me - the only episode that deserve a 10/10 it's Come Undone, even if thinking about it, I love them both at the same way.
Review of The Man in the Mirror
