Coming up in the next paragraph, as River Song would say: "Spoilers!"
Lindsay Duncan plays Adelaide, the head of Bowie Base One, the first off planet colonization of humans. The thing is, like Pompeii, what happens on November the 21st, 2059 is a set point in time. The Doctor can't interfere. Of course, he did interfere with Pompeii, and of course it turned out that he had to cause it. But the chain of events is already in motion here. The Doctor must leave. Whatever caused the destruction on Bowie Base on this day must be allowed to happen. Adelaide's death is a fixed point that will shape history. But the Doctor makes a terrifying choice, and choice that could change everything. The Doctor is who he is because he obeys the laws of time, the laws of his people. But he comes to the conclusion that if he is the last, there's no one there to say he's wrong. He decides to save those he can. The horror he faces as he walks away and hears the agonizing death screams of the crew make him snap. Even though he has to walk away, he doesn't. This one act leads him to be above all, to no longer be a monitor of time, but the master of it. Nothing stands in his way... he could save Rose, he could save Donna, he could no longer be alone. Nothing stands in his way except Adelaide. She who realizes the mania that has taken ahold of him. She who realizes that he thinks he's better than everyone. She stops him.
With Adelaide's death the Doctor realizes that he has gone too far. He was on the path to becoming the master. The fear of his impending death has him so scared that he's lashing out at time itself. He knows the inevitable is here. I think this is the best special so far, in that it actually tells an important story. This is a Doctor we've never seen before. He never turns his back. We see the desperation that has gripped the Doctor. He knows he can't stop what's coming, and though he should have left this fixed point on Bowie Base alone, the mania that gripes him seems to indicate that he believes that perhaps, if he can change what should and couldn't be changed, perhaps his future will be different as well. But when the realization hits that it's the end of the line, as he kneels in the snow and pleads to Ood Sigma... I hope his final story is worthy of what a great Doctor he was.
Also for you Doctor Who freaks out there, anyone else notice that one of the crew was also on a Sarah Jane episode? "Warriors of the Kudlak" anyone? And if you religiously watch Confidential... Russell is SO wearing Steed's coat from The Avengers, and Danny Hargreaves eventually makes an appearance near the end describing the 14ft explosion he rigged. And as a final treat... the trailer for the end...