But the true bad guy of this story isn’t the Zygons: It’s the long-unmentioned (OK, retconned) version of the Doctor played by John Hurt.
At the 2005 restart of the series, previous showrunner Russell T. Davies wrote out the Doctor’s mostly pompous and boring people, the Time Lords, by having the Doctor destroy both them and their archenemies the Daleks to bring an end to the Last Great Time War. While most people assumed that it was the Eighth Doctor who had fought in the Time War, and then regenerated soon after into the Ninth (c.f., Christopher Eccleston checking himself out in a mirror in his first episode and exclaiming, “Look at the ears!”), current executive producer and head writer Steven Moffat inserted a character of his own invention into that blank space, Hurt’s “War Doctor.” It turns out that Hurt’s guy was the one who used a weapon called The Moment (first referenced in “The End of Time,” the final episode for both Tennant and for Davies as showrunner) to destroy both sides in the Time War.
In addition to throwing his own version of the Doctor into this wartime scenario, Moffat also added an intriguing twist: The Moment is sentient and has a conscience. Reaching into his future memories (it’s a Time Lord invention, pretty easy to accept), The Moment appears in the form of Rose Tyler, companion to both the Ninth and Tenth Doctors.
Even though she isn’t the real Rose, she acts like her, playfully mocking Hurt’s portentous declaration, “No more.” (She always used to take the piss out of Nine and Ten like that.) And in a weird way, Moffat’s choice here—that an entity trying to steer the Doctor towards becoming his best self would show up as Rose—confirms her status as the most important companion he’s ever had.
So, Moment/Rose acts as Ghost of Doctors Yet to Come to Hurt’s very Scrooge-like War Doctor: She creates time tunnels to drop him in to meet not only the Eleventh Doctor, who’s investigating the back end of the Zygon plot in the 21st century, but the Tenth, who’s on the trail of its inception in the 16th.
For lots of fans, it’s Tennant’s presence that makes this special a special. And he has plenty of fun moments in this one, including using an improvised machine that goes “ding” and making the old “Oncoming Storm” speech at a fluffy bunny rabbit. Whether he’s being funny-technobabbly Doctor or Time War-angsty Doctor, he’s a joy to watch.
But ultimately, his role here is the most fan-servicey. We know exactly where he is in his own arc—procrastinating facing his doom between “The Waters of Mars” and “The End of Time”—and we know, as Ood Sigma would put it, how his song ends.
The best part of any multi-Doctor story is always watching the Doctor talk to himself. Two and Three bickered quite amusingly—and believably—in both “The Three Doctors” and “The Five Doctors.” Smith and Tennant go in for a bit of that sort of banter, in a predictably Moffaty assign-a-nickname-based-on-physical-appearance way (“Chinny” just isn’t very funny), and also measure their dicks, er sonic screwdrivers (more Moffatry). But mostly they get along quite well.