Children In Need
The first full adventure to star David Tennant as the Tenth Doctor was preceded by a seven-minute mini adventure during the BBC's 2005 Children In Need appeal. As well as giving us our first extended glimpse of the Tenth Doctor in action, the special was also the first time in the new series that the sound effect of the TARDIS's warning system, the Cloister Bell, was used.
Song For Ten
A special piece of music, Song for Ten, was composed by Murray Gold for the episode. Sung by Tim Phillips, it was the first original song written for the series since The Ballad of the Last Chance Saloon in The Gunfighters (1966).
The Doctor refers to another Science Fiction icon, Arthur Dent. Created by former Doctor Who script editor Douglas Adams, Dent was the star of international publishing, radio, TV and film phenomenon The Hitchhiker's Guide To The Galaxy. The Doctor has previously referenced The Hitchhiker's Guide to the Galaxy in Ghost Light (1989), and also mentions a book (The Origins Of The Universe) by HH character Oolon Colluphid in Destiny of the Daleks (1978).
British Rocket Group
The British Rocket Group, responsible for the Guinevere probe, was first mentioned in Remembrance Of The Daleks (1988). The name is a reference to the British Experimental Rocket Group from the classic BBC Quatermass serials. Davide Tennant and actor/writer Mark Gatiss starred in a live TV performance of Quatermass not long before Tennat's TV debut as the Doctor.
The Doctor's right hand is severed during the story's climactic swordfight, revealing a previously unknown Time Lord ability. It seems that they can fully restore damage to their new bodies within fifteen hours of their latest regeneration cycle. The severed hand later went on to cameo in spin-off series Torchwood, before returning home with Captain jack in Series Three's Utopia.
The Christmas Invasion was the first Doctor Who episode to premiere on Christmas Day since The Feast Of Seven, part seven of The Daleks' Master Plan (1965). It was also the first episode to premiere on a Sunday.
Attack Of The Graske
Immediately after The Christmas Invasion aired on TV, viewers with digital TVs were able to use their 'red button' to take part in a special Tenth Doctor interactive episode, Attack of the Graske, written by Gareth Roberts.
'Sycorax' is taken from name of the witch in Shakespeare's play The Tempest. In Series Three's The Shakespeare Code, the playwright takes note of the Doctor's reference to the name, thus creating something of a paradox as to where it originated.
The music for the closing credits of the episode restored the traditional middle eight section of the theme tune, absent in the music for Series One. The theme was performed by the BBC National Orchestra of Wales.
At his request, David Tennant was credited as 'The Doctor' rather than 'Doctor Who'. Christopher Eccleston had been credited as the latter, as had all Doctors prior to Peter Davison's Fifth incarnation.