09. The Waters of Mars (S4, Russel T. Davies and Phil Ford)
I'll spoil something right away that probably isn't really much of a spoiler at all: this is the only RTD entry on this list, but damn, it's amazing. Normally, I'm not really a fan of RTD's style, and this placement probably only confirms this, because this about as far away from his own style as RTD ever got - this and Midnight, maybe.
Anyway, what makes this so special? It starts out pretty basic - people in space are attacked by an alien lifeform, in this case a virus, that picks them off one by one. The way that the virus spreads is pretty interesting, and the visuals of what it does to people are pretty creepy - especially those cracked mouths. Adelaide Brooke is also an interesting companion for the Doctor - well, not really a companion, but that's the closest to it that it gets here - very headstrong, like Donna, to an extent, but a totally different character. She doesn't come across as the most likeable person at first, but you quickly sympathize with her plight.
While the basic story is pretty straight-forward, one thing is very different here: the Doctor. When he arrives at the Mars base (named, wittily enough, Bowie Base One), he quickly realises what he is dealing with: a fixed point in time, something that can never be altered, by anyone. While he tells himself to leave all the time, because there is nothing that he can do, he doesn't really do anything to actually leave and it's quite evident that he is struggling with himself, and that he is pretty interested in what caused Adelaide to detonate the nuclear device.
Where the episode really picks up, however, is about fifteen minutes to the end. The Doctor finally leaves the Base and walks to the TARDIS across the red planet, as the Base explodes behind him, and he hears the screams of pain of everyone through his communicator. While this is certainly an extremely powerful scene, it's next to nothing in comparison with what comes next.
The Doctor snaps. Plagued by all the losses he has suffered, he decides no more and returns. What follows is actually pretty standard-fare - the Doctor saves the day in a creative manner - but somehow, it all feels wrong. Again, Tennant is a large part of why this is so amazing - his slightly demented facial expression, that becomes even more crazy all the time is as unsettling as it is brilliant acting. And it only gets better/worse - on Earth, the Doctor snaps completely, displaying arrogance that he so rarely shows, proclaiming himself as the Time Lord Victorious - the Laws of Time are mere playthings to him, and he has complete dominion over time.
Until Adelaide ends it all with one shot of her gun. The Doctor breaks down completely, recognizing that he has gone to far. Ood Sigma appears, like The Watcher in Logopolis, signalling that the end is near. However, the Doctor remains defiant and leaves with a defiant "No!".
While that was basically just a summary what I just wrote, I really couldn't describe what I love about this episode any better or any shorter. Again, Tennant is the star here, displaying a demented and crazy side to him that we have never seen, and it is arguably the most powerful moment of the RTD era on the show. To see Tennant snap in such a spectacular fashion, only to be brought back down to Earth so harshly... alone that is worth the inclusion in the Top 10. The rest, while good, is just the icing on the cake - the last ten minutes are arguably some of the best ten minutes of Doctor Who, ever, in my opinion.