WARNING, MANY SPOILERS

David Tennant has had a fabulous run as The Doctor, and now the series is winding towards his closing time.  Although "The Waters of Mars" doesn't directly lead into the last two Tennant episodes (aside from a tiny bit at the end) it represents a change in tone for the series, from "sprightly and fun" to "downright funereal."

The Doctor is travelling alone in "The Waters of Mars," and that never ends well for him.  There's a reason why The Doctor chooses a companion.  In this special, we get to see full well what happens when he goes banging about without an assistant to balance out the crazy and act as a moral compass.  Frankly, he goes completely off the rails.

But I'm getting ahead of myself.  The Doctor lands on Mars, dons his orange space suit, and toddles off to a nearby Mars station.  The station residents, a collection of scientists working on the first human beachhead on Mars, don't seem nearly as unnerved by The Doctor showing up on their doorstep as they ought to be.  They are startled, to be sure, but they quickly write him off as being part of another country's surprise landing next door.  At any rate, they soon find something else to distract them away from wondering who The Doctor is, because water monsters are turning the crew into zombies.

If this had been any other episode, I would have said that the water zombie monsters were pretty weak, even by the standards of a one-off Doctor Who episode.  They don't really do much, aside from take over the crew member's body, and stand around with water pouring out.  But what this lacks in ACTION EXCITEMENT it more than makes up for in quiet yet inexorable menace, which happens to be this episode's orders of the day.

The real horror of "The Waters of Mars" comes from watching The Doctor stand about, waiting for everyone to die, and refusing to help because he feels that their deaths are "fixed points in time."  Once he establishes the date, he understands that the station is going to nuke itself before the day is through, with all crew members lost. Furthermore, this has to happen, in order for other important future events to take place.  For once, The Doctor refuses to act.  

Instead, he stands around looking sad and brandishing his Prime Directive about.  And it is horrible to watch.  We realize how accustomed we have become to the idea that The Doctor will swing in and intervene in any event, no matter how ill-advised or ridiculous.  That's part of his charm.  The scene where he walks slowly away from the Mars base while the panicked last messages of the crew members play over his intercom system is a thousand times more horrifying than any monster ever could be.  

When The Doctor does finally act, it only makes us feel worse.  He takes charge of the events with the sudden conviction that he himself is the sole ruler of time and space.  That can't be good, we think, and we are right.

Watching "The Waters of Mars" was such a grueling experience that I feel I am going to really have to gird myself to watch the next two episodes.  Russell T. Davies is setting us up for a massive blow-out on this one, make no mistake.