The Eighth Doctor Adventures novel Interference: Book Two by Lawrence Miles indicates that the Doctor built a Mark IV model sometime prior to the events of that book, but what happened to this unit is not stated. In the novel The Gallifrey Chronicles by Lance Parkin, K9 Mark II makes another appearance, having been trapped within the TARDIS since the events of The Ancestor Cell. At the end of The Gallifrey Chronicles the Doctor sends K9 on a secret mission to Espero, presumably to seek out his former companion, the living TARDIS known as Compassion.
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Although John Leeson has been the primary voice actor for K9 since his introduction, two other actors have voiced the character. David Brierley voiced K9 during Season 17 after Leeson chose not to return to the series (Leeson subsequently signed up again for Season 18), and Roy Skelton, better known for his work voicing Daleks, voiced the character in Destiny of the Daleks when all was required of K9 was a croaking sound due to the character contracting a form of robot laryngitis.
Almost as soon as he arrived, K9 Mark IV left Sarah Jane to stabilise a dangerous black hole in space which had been created in Switzerland. He could only speak with Sarah Jane occasionally via a cabinet with a space-time link when the orbit of the black hole intersected with Earth's and did not know when he would complete the job. (TV: Invasion of the Bane) However, he returned to Earth very briefly during emergencies. He did so on two later occasions in order to render vital assistance. The first time, he fought Mr Smith in pitched battle to give Sarah Jane time to upload the Armageddon Virus. (TV: The Lost Boy) The second time, he quickly uploaded TARDIS base codes to Mr Smith, assisting the Tenth Doctor in returning Earth to its correct location in space. (TV: Journey's End)
On 24 April 2006, The Independent, the Daily Star and The Times confirmed, following previous rumours, that K9 would be featured in a 26-part children's series, K9, to be written by Bob Baker. The article in The Times also featured a picture of the redesigned K9 for the animated series. The series will be a blend of live-action and a CGI K9.
K9 Mark I's mechanisms included rotating ear-probes, telescopic "eye" probe, an extendable nose concealing a powerful laser weapon, flashing lights on the top and the "eye" panel, waggling tail antennae, and ticker-tape tongue. His head could move up and down. His shell was painted in metallic gold/grey. There was a monitor screen on his left flank, though K9 rarely used this during his travels with the Doctor. Around K9's neck was a tartan collar, (being the Family Tartan of the Clan Buchanonn) from which hung a silver disk. (TV: The Invisible Enemy) He was later modified to fly through the vacuum of space. (TV: Curse of Anubis)
Whereas Sarah Jane lifted K9 Mark III into and out of her car, (TV: A Girl's Best Friend) K9 Mark IV had a "hover mode" allowing him to negotiate the stairs of the Smiths' three-story house and bi-level attic, and to enter and exit cars. (TV: The Wedding of Sarah Jane Smith, The Gift) The Eleventh Doctor implied that Mark IV was not the first to have the ability to hover. (TV: The Power of Three)
When Romana eventually returned to her own universe, she brought K9 Mark II with her (presumably having repaired it) and eventually became Lady President of Gallifrey. K9 Mark II is first seen on Gallifrey in the Virgin New Adventures novel Lungbarrow by Marc Platt, alongside K9 Mark I, which had remained with Leela. Both K9 Mark I and Mark II appear, voiced by Leeson, in the Big Finish Productions audio adventure Zagreus and the Gallifrey audio series. Leela's K9 (Mark I) was destroyed at the conclusion of the second Gallifrey series and only Mark II appears in the third series.
K9, retroactively "K9 Mark I" (Leeson), initially appeared in The Invisible Enemy (1977) as the creation of Professor Marius (Frederick Jaeger) in the year 5000. K9 subsequently travelled with the Fourth Doctor (Tom Baker) and Leela (Louise Jameson) as a companion of the Doctor in his adventures in time and space until The Invasion of Time (1978). In this serial, K9 decides to remain on the Doctor's home planet of Gallifrey with Leela. Immediately afterwards, Doctor Who would introduce a second incarnation of K9, played by the same prop; the last scene of The Invasion of Time shows the Doctor unpacking a box labeled "K9 Mk II".