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.
K9 Mark IV was a recreation and enhancement of the Mark III created by the Tenth Doctor as a second present for Sarah Jane. (TV: School Reunion) This model continued in service to his mistress, Luke Smith, Clyde Langer, and Rani Chandra and was known by Maria Jackson and Kelsey Hooper. He was left fixing a black hole in Sweden for many years, but after returning to Earth, was used sparingly, until he went to university with Luke Smith. (TV: The Nightmare Man)
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K9 was not originally intended to be a companion, but producer Graham Williams liked the concept so much that the decision was made to retain him as a regular character, in order to appeal to the younger members of the audience. The original name for the character was "FIDO" — apparently from "Phenomenal [sic] Indication Data Observation" unit — but it was eventually named K9.
Having left K9 Mark III to Sarah Jane (TV: A Girl's Best Friend) following their travels with Adric in his fourth incarnation, (PROSE: Just a Small Problem) the Doctor eventually built K9 Mark IV. The Eighth Doctor mentioned Mark IV when he met Sarah Jane in 1996. He indicated that the effort in constructing Mark III was "nothing" in comparison to Mark IV. (PROSE: Interference - Book Two)
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)
During Romana's Presidency of Gallifrey, Leela became her bodyguard, and the two K9 units were forced to work together, not always amicably. After Matthias had replaced Romana as President, he nominated her to undertake a mission to find a cure for the Free Time virus, to be followed by exile. K9 opted to remain in the employ of Matthias, fighting the infection on Gallifrey. However, Braxiatel and K9 had planned another operation. Realising that the only cure to the virus was to end the regeneration capabilities of the Time Lords, Braxiatel used a Time Scoop to collect the Time Lord bio-data archive (along with K9). Romana was asked to decide whether to administer the cure or let Gallifrey die and rebuild it from scratch with the bio-data archive. (AUDIO: Gallifrey)
Professor Frederick Marius, who invented the first K9 in the year 5000 while working on the asteroid K4067, described him as "my best friend and constant companion." Marius had a dog on Earth, but weight requirements did not allow him to bring his real dog into space, so he built K9. (TV: The Invisible Enemy) He built K9 by using the latest technology. Marius used his own medical computer, a state-of-the-art intraresponsive brain app, protective anti-radiation cladding, probes, a laser scapel, a vision and voice box and two scanning antennas. He put these parts together and created K9. (PROSE: One Man and His Dog) According to one account, the Professor copied the remnants of one of the versions of K9 created by the Doctor and given to Sarah Jane Smith. (PROSE: Tautology)
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A third model, K9 Mark III was evidently constructed by the Fourth Doctor; indeed, the Eighth Doctor did not dispute the presumption that he had built this K9. (PROSE: Interference - Book Tqo) Mark III travelled with the Fourth Doctor and Adric for a time. (PROSE: Inter-Galactic Cat) Eventually, however, he was shipped in 1978 to Sarah Jane Smith at the home she had shared with her Aunt Lavinia in South Croydon during her companionship of the Third and Fourth Doctors. (TV: A Girl's Best Friend) Since parting ways with the Doctor (TV: The Hand of Fear) however, Sarah Jane had already gone away and was travelling extensively. The following year, Lavinia purchased a manor house and market garden in Moreton Harwood, Gloucester, and took the still crated K9 with her. On 18 December 1981, Sarah Jane and her foster brother Brendan Richards arrived at Lavinia's home for Christmas, but Lavinia had left early for her lecture series in the United States. Sarah Jane opened the crate and received her new companion. (TV: A Girl's Best Friend)
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K9 Mark III first appeared in "A Girl's Best Friend", the 1981 pilot for a series, K-9 and Company, which never materialized beyond the first episode. In this, the character is presented to the Doctor's former companion Sarah Jane Smith (Elisabeth Sladen); together, the two embark on what a number of adventures. Only the first of these is depicted on-screen; several are shown in comic book and audio play adaptations, and others are alluded to by the television series. Mark III made a brief appearance alongside Sarah Jane in the Doctor Who 20th anniversary ninety-minute special The Five Doctors (1983), before appearing properly and for the final time in the revived series' episode "School Reunion" in 2006. By the time of that story, Mark III has fallen into disrepair and eventually sacrifices himself to stop a plot by the alien Krillitane and defeat their leader, Lucas Finch (Anthony Head). In the episode's conclusion, the Doctor presents Sarah Jane with a new K9 to encourage her to continue investigating alien activity; the Doctor "rebuilt" him after the Mark III's sacrifice, implying that he had the same mind and memories as his predecessor while still being a "brand new model".