The Fourth Doctor found that K9 had contracted laryngitis and had lost his voice as a result of this. K9 was left in the TARDIS while the Doctor and Romana II battled the Daleks and Davros. (TV: Destiny of the Daleks) During a later adventure, he remained patiently in the TARDIS, which would imply that his voice had not yet returned. (TV: City of Death) After his voice returned, it sounded quite different and remained so for a while (TV: The Creature from the Pit) before the familiar voice returned. (TV: The Leisure Hive)
When the Tenth Doctor created K9 Mark IV to replace K9 Mark III, he added a shiny metallic blue paint job and other improvements, including an omniflexible hyperlink facility. (TV: School Reunion) Later, though, K9 reverted to his old dark green colour. (TV: Invasion of the Bane) In battling Mr Smith (and following the battle) K9 displayed a teleport facility and a laser weapon similar to the previous K9s'. (TV: The Lost Boy)
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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.
K9 was a popular enough character to warrant an attempt to spin him off into his own series. In addition to K9 dolls, there were also talking K9 toys produced by Palitoy, the speech provided by a miniature record inside the body of the toy. John Leeson provided the voice for K9 in all its appearances, except for Season 17 (which included the unfinished Shada) when it was voiced by David Brierley. When Shada was remade by Big Finish Productions, Leeson provided its voice.
Someday K9 saved the people at the Aurora from being killed by a Federation war cruiser. (PROSE: Peaceniks) When K9 wanted to help a space ship navigating back to New Earth he met Interstitials. These creatures were living in a state relating to the space or time between things. K9 helped them pass into another existence where it was better for them. (PROSE: Interstitials)

When being imprisoned by the Sporraxx they tried to access K9's memory core. Memories that K9 had assumed to be erased were slowly coming back and the Sporraxx got to know more and more about K9's past. They reprogrammed K9 to protect a small spherical marauder bomb, that would destroy Earth. However K9 was able to break free of their control, because deep in his memories a sense of right and wrong prevailed. He took the bomb back into the Sporraxx space ship where it exploded. (COMIC: Short Circuit)

Unlike 25 years earlier where she appeared unconcerned with K9 Mark III being seen, Sarah Jane restricted K9 Mark IV to her Ealing house except in emergencies or when she required his services, lest she have to explain him. Her son Luke covered by explaining to a puzzled Peter Dalton that K9 was a Japanese robot toy with pre-programmed phrases. (TV: The Wedding of Sarah Jane Smith) Nevertheless, she sent K9 to serve Luke when he went away to the University of Oxford. (TV: The Nightmare Man)


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.
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