"The Christmas Invasion" is a story of firsts for Doctor Who. As well as introducing a new alien species to the Whoniverse—the warrior race known as Sycorax—it was the first in a brand new tradition, the Doctor Who Christmas Special, it introduced the threatening Torchwood institute and, almost incidentally, it was the first full story for David Tennant's Tenth Doctor, a regeneration special that fiddled with Time Lord mythology just enough to allow the Doctor to recover from being maimed without undue trauma.

Here are a few things that you should keep an eye out for, the next time you watch.

(The episode is available on iTunes and Amazon.)

Before "The Christmas Invasion," only one episode of Doctor Who had made its first appearance on Christmas Day (and as Christmas Day was on a Sunday in 2005, it's worth noting that no episodes had ever premiered on a Sunday either). This was "The Feast of Steven," part seven of the 1966 First Doctor 12-parter "The Daleks' Master Plan," which also features one of the only moments in which the Doctor turns to address the audience at home. As recreated here in a teaser for Mark Gatiss's "An Adventure in Space and Time":

https://www.youtube.com/watch?v=ox8QYFvltIw

The Sycorax were named after the witch who gave birth to the malignant beast Caliban in Shakespeare's The Tempest. As a character, she's never seen on stage, but the wizard Prospero describes her as a "blue-eyed hag" who was left on his magical island by sailors. She immediately traps the spirit Ariel within "a cloven pine" for twelve years, and then dies, leaving Caliban "a freckled whelp hag-born—not honour'd with a human shape" to fend for himself.

Of course, in the Whoniverse the inspiration runs the other way, as the Doctor spots a skull that reminds him of the Sycorax during "The Shakespeare Code," prompting the bard himself to pinch the name for future use.

Although it seems like a natural extension of Rose's emotional state after the regeneration, the reason the episode is set back at her mum's flat is to do with Doctor Who's new audience. Russell T Davies wanted to make sure the reality of the show hadn't been too badly affected by the departure of Christopher Eccleston, especially given that the series had only just come back, so set his first regeneration story in London, bringing back Rose's mum Jackie, her ex Mickey and Harriet Jones as a way of reassuring everyone that it was business as usual.

https://www.youtube.com/watch?v=oF2EmKdTbsc

When writing the names of the characters who were affected by the Sycorax's blood control, Russell T Davies raided his own family for inspiration. A mother is seen begging her two children Catherine and Jonathan to wake from their spell. Those are the names of Davies' nephew and niece.

Davies also wrote a line to explain why the Doctor lost his Manchester accent when he regenerated (and, for the viewers, why he doesn't speak with David Tennant's Scottish accent). The Doctor claims that his new mode of speech is derived from Rose's, "like a chick hatching from an egg." Although to a trained British ear, David's accent as the Doctor is a couple of notches up the class ladder from Billie Piper's Estuary English accent as Rose. He's closer to Harriet Jones's Recieved Pronunciation than the daughter of Jackie Tyler.

The Doctor responds to his welcoming committee on the Sycorax ship by saying "Blimey, it's like 'This Is Your Life'!", a reference to the TV show that ran on British screens from 1955 until 2003 (on and off). A true moment of trans-Atlantic exchange, the British version was based on the American show of the same name, in which celebrities were surprised with a book of their life history, and special guest appearances from some of their best friends. It started on NBC radio in 1948, transferring to TV in 1952-1961, with more shows in 1971-72 and some specials in the 1980s.

https://www.youtube.com/watch?v=YtesZDblpgA

When putting together the Doctor's eventual final costume—after the scene in which the Tenth Doctor is seen considering the frilly shirt David Tennant wore in Casanova—designer Louise Page wanted to pair the pinstripe suit and long riding coat with a pair of Japanese army boots. David Tennant said he wanted to use his own sneakers instead, to undercut the smartness of the suit with casual shoes. And also to aid all of that running Doctors do.

As the new Doctor is seen searching the wardrobe of the TARDIS for a new outfit, the Fourth Doctor's long multi-colored scarf can be seen (among lots of other outfits reminiscent of his previous costume choices) on a hanger. This had been knitted specially by the great-aunt of series producer Phil Collinson.

https://www.youtube.com/watch?v=vu1C1Wmj3Xc

If you listen to the music in the closing credits—as performed by the National Orchestra of Wales, conducted by Murray Gold—this is the first time in the new series that the theme tune appears with its traditional "middle eight" section. It's part of the theme music that hadn't appeared during the first season, but would play out during all the episodes of Season Two.

But for this episode's most obscure reference chain, we turn to the Guinevere One project, the space probe carrying human blood that allows the Sycorax to control so many of the population. There was a tie-in BBC website created for this episode, which stated that the probe was created by the British Rocket Group, a reference to an organization first mentioned in the 1988 Seventh Doctor story "Remembrance of the Daleks." This was, in turn, a reference to the Quatermass science fiction stories, which featured a British Experimental Rocket Group.

Which might seem too obscure to bother mentioning, had David Tennant not just appeared in a live remake of The Quatermass Experiment as Doctor Gordon Briscoe. During the broadcast, the first line addressed to his character was subtly changed from "Good to have you back, Gordon" to "Good to have you back, Doctor" a mere two weeks before David was formally announced as Christopher Eccleston's replacement. Coincidence? Who can say?

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