Doctor Who wrestles with fate in "The Waters of Mars"

[Here there be spoilers. You have been warned.:]

I have to be honest: I've generally been a bit underwhelmed by the between-seasons Doctor Who specials. They're always fun, but they've generally felt a bit lackluster compared to the full seasons. I'm happy to report that "The Waters of Mars" is easily the best of the specials yet. It's also quite possibly the bleakest Doctor Who story ever produced. And both of those qualities are directly linked to the way in which the story tackles...