Argh. Late again, chalk it up to a faulty connection (I hate my ISP), and having to service my comp (dirty fan).
But WHOA. What a kicker. I definitely must agree with Grog, this is the best script Russell T. Davies has written for Doctor Who. He takes the standard plot trappings of the show (base under siege, humans in danger, and the Doctor saving the day), and turns it on it's head. The result is fantastic.
And what was best here is that while there is a monster of the week, it wasn't the scariest thing in the story. What was scary was the collective paranoia and fear that gripped the passengers, so much so that they were willing to do the unthinkable. It's stuff like that the Doctor has no answer to, and the episode really portrays his helplessness at the situation. Lovely acting from David Tennant, but I must commend all the guest stars, especially Lesley Sharp, upon whose creepy performance the success of this entire episode revolves.
In conclusion, I'd rate this as one of the darkest stories of Doctor Who ever, alongside The Caves of Androzani .