The initial idea for realising K9 was to use a small actor inside a robotic Dobermann costume, but that was rejected in favour of a radio-controlled prop, designed by Tony Harding and made by the BBC Visual Effects Department. The Radio Control Model Centre in Harlington Middx owned by Derek Wales was commissioned to build the electronics into the first original basic shell and consequently the centre operated the dog on set for the BBC.[citation needed] The robot suffered from numerous technical problems during its time in the series, often malfunctioning because the radio controls interfered with the cameras and vice versa. On location, K9 also proved unable to traverse uneven terrain, and shots had to be conceived with this in mind. Workarounds included using a concealed piece of twine to pull the character along (this string can be clearly seen in a shot of K9 on Brighton Beach), or laying wooden planks on which it could roll.
The Virgin New Adventures seventh Doctor novel Lungbarrow (written by former Doctor TV series writer Marc Platt) featured K9s Marks I and II meeting for the first time on Gallifrey during the events surrounding the disappearance of the Doctor's entire family house and his living relatives, which also featured many previous established Gallifreyan based characters. The two collaborated towards rescuing the Doctor, then current companion Chris Cwej and the Doctor's lost family. This novel also served as the direct lead-in story to the 1996 Doctor Who telemovie.
Before this, the Doctor obtained or constructed at least one backup model of K9. Immediately after leaving Mark I behind with Leela, he unpacked K9 Mark II. This version of K9 accompanied the Doctor and his new companion on their quest to locate the segments of the Key to Time, (TV: The Ribos Operation) but the swampy terrain of Delta III made it impossible for him to join them on the planet. At the conclusion of the quest, the Shadow turned K9 to his evil will, giving K9 a sinister personality. (TV: The Armageddon Factor)
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.
This K9 exhibited the ability to sense and warn others of danger. He was also more mobile than his predecessor, despite his problems with the swamps of Delta III, able to lift himself up on small support rods so that he could climb stairs (PROSE: The Shadow of Weng-Chiang, TV: The Power of Kroll, The Armageddon Factor) The Doctor often used a glib remark to disarm those who were surprised by K9's appearance, such as "They're all the rage in Trenton, New Jersey." (TV: The Stones of Blood)

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)
K9 returned when Ship used the black hole as fuel. Though Sarah Jane, Luke, Rani and Clyde were happy to have K9 back, Mr Smith was not. (TV: The Mad Woman in the Attic) K9 spent a lot of his time in Sarah Jane's attic at 13 Bannerman Road, Ealing, but left to defeat Travast Polong. He attended Sarah Jane's abortive wedding to Peter Dalton, where he helped the Tenth Doctor, Luke, Rani and Clyde defeat the Trickster. (TV: The Wedding of Sarah Jane Smith) After helping defeat the Slitheen family, Clyde brought K9 to Park Vale Comprehensive School to help him with his test. He helped defeat the Rakweed before returning home. (TV: The Gift) K9 went to the University of Oxford with Luke, but restored Mr White's original programming through a mobile phone.(TV: Goodbye, Sarah Jane Smith)

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".
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