The BBC Press Office has issued a full press release on their Christmas offerings, noting "this year, the BBC offers a mix of new Christmas specials alongside some established favourites." The press release itself notes the rerun of Casanova on BBC3... and gives a full preview of the forthcoming Doctor Who special on December 25. They also note that there will be some Doctor Who content on BBC Three that day, though it's unspecified.
Below, in the spoiler tags, is the BBC press synopsis for "The Christmas Invasion"; it gives the plot setup away so don't click it unless you want to know!
It's Christmas and Jackie Tyler is at home preparing for the festive season, not knowing if her daughter Rose will be home in time - or at all. Then she hears the familiar, ancient grind of the TARDIS! Mickey, who is hard at work, also hears it and they both race across the estate just in time to witness the TARDIS' glorious, chaotic crash landing. The doors open and, much to their delight, out steps Rose and a complete stranger - except it isn't a stranger - it's the Doctor. Disorientated, yet overwhelmed to see them, he takes a few minutes to regain his balance and consider what it is he wants to tell them. "Oh! I know! Merry Christmas!" he yells, before collapsing. The Doctor falls in and out of consciousness, with Rose and Jackie powerless to help him.
In a bid to boost Rose's spirits, Mickey suggests a spot of Christmas shopping. A good plan - until they find themselves under attack by a sinister brass band of masked Santas. However, they are not the only ones with problems. Prime Minister Harriet Jones has just been informed that a British space probe, on its way to Mars for a Christmas Day landing, has gone missing. It has been kidnapped by a monstrous race known as the Sycorax, who are set on taking over the world. Meanwhile, back at the Tylers', Mickey and Jackie are trying to fend off a killer Christmas tree, while Rose tries to wake the Doctor.
David Tennant plays the Doctor, Billie Piper plays Rose Tyler, Camille Coduri plays Jackie Tyler, Noel Clarke plays Mickey, Penelope Wilton plays Harriet Jones and Sean Gilder plays the Sycorax leader.