Stepping back from Whoniverse lore, and back to this episode, here the audience gets to see what happens when the Doctor comes face to face with an unchangeable event. It is an interesting question and explored here in great fashion (though telling you how would significantly hurt your enjoyment of the episode).
"The Waters of Mars" features a good performance by Lindsay Duncan as the gruff Brooke, as well as solid outings by the actors whose characters comprise the Bowie Base One crew. Tennant delivers another outstanding performance which makes one both incredibly excited at the prospect of the next two Doctor Who specials ("The End of Time" parts one and two) airing, and incredibly sad as they will be Tennant's last trips in the TARDIS.
For now though, with "The Waters of Mars," we have been given exactly what a Doctor Who special should be. It functions beautifully as a sci-fi horror genre piece and manages to expand beyond those confines into loftier issues without ever losing the core thread.
Doctor Who – "The Waters of Mars" airs at 9:00pm on BBC America, December 19.