The Tenth Doctor’s time is growing short. As promised by outgoing showrunner Russell T Davies, things begin to get dark here. That’s an understatement.

2059. The Tenth Doctor (David Tennant) arrives on Mars at the first human outpost on the planet, Bowie Base One, under the command of the formidable Adelaide Brooke (Lindsay Duncan), just as terrible events begin to occur. Only he knows the truth of how it will end, and the laws of time forbid him from stopping it.

As written by Davies and Phil Ford (a previous contributor to spinoffs The Sarah Jane Adventures and Torchwood), it’s a compelling hour of drama for Who fans. At its core, it’s a story about choices and their consequences, invested with a sense of inescapable tragedy that begins at a low level and slowly climbs to a wrenching climax. While it’s not quite in the same league as the brilliant Torchwood: Children of Earth, it does share a similar theme and tone, and Davies is a writer who always works best on an emotional level (something his critics so often misunderstand).

Veteran director Graeme Harper, whose involvement in the show dates back to 1966, confidently renders the story on screen with maximum suspense and some strong performances. In grand Who tradition, the exterior Mars scenes were shot in a quarry, but the CG effects render it believable. The monsters of the week are effective, and much like director Hideo Nakata did in the film Honogurai Mizu No Soko Kara, Harper turns even a single drop of water into a source of dread. I’ve read complaints that some viewers wanted more explanation of what the monsters were and what they were after, but that’s really missing the point: the monsters are a classic MacGuffin, used to setup the Doctor’s later actions. It’s all about the Doctor’s emotional journey here.

What we’re left with at the end is the idea that there are some lines a Time Lord (not even the last of the Time Lords) shouldn’t cross, and doing so will have terrible consequences for the Doctor. The past three seasons have offered numerous stories of the Tenth Doctor operating as an almost messianic figure with great powers. Now we’re beginning to get the payoff…and, oh, is it a doozy. Davies has clearly planned this all along, with the Tenth Doctor increasingly abusing his Time Lord powers for good ends and slowly creeping toward a line that should have remained uncrossed.

Tom Baker will always be my favorite Doctor, but Tennant is a close second because he’s the best actor to have played the role, and he delivers a bravura performance in this special. His expressive face details every emotion the Doctor goes through, from joy to sadness to a frightening arrogance to horror at his own actions. We see aspects of the Doctor that we’ve never seen before, or at least have only been hinted at. After all the horror and death the Doctor has seen since the Time War, he finally cracks, if only for a few minutes, but that’s long enough to make a serious error.

Duncan is marvelous as Adelaide, displaying humanity, grace, and courage. Much like Lady Christina in the previous special, Adelaide would have made an amazing companion for the Doctor under other circumstances. Tennant and Duncan play especially well off each other, adding to the emotional depth of the story.

I enjoyed “The Waters of Mars” very much, even if I felt uncomfortable with where it was taking the Doctor, which is precisely the effect it needed to have. The stage is now set for the two-part “The End of Time,” which is sure to be a brilliant finish for David Tennant as the Doctor and Davies as showrunner.

(And I loved GADGET!)