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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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)
After K9 Mark I regenerated into K9 Mark 2, his design was radically altered. His ear probes were replaced with larger silver aerials. A set of blue lights on his head flashed when he spoke. Most of his body was dark silver and a silver dog bone was located under his neck. He seemed to have the same capabilities as Mark I but was presumably more advanced. (TV: Regeneration)
K9 had a personality very consistent across the models with some contact with the Doctor. He was polite and formal, with pedantry bordering on condescension. Though he often displayed feelings such as sorrow (TV: The Invasion of Time) and self-regard (TV: School Reunion), he often stated that he had not been programmed to have emotions. (TV: Invasion of the Bane)

K9 Mark I was created about the year 5000 by Professor Frederick Marius, who gave the dog to the Fourth Doctor and Leela. He was left behind with Leela on Gallifrey. (TV: The Invasion of Time) Some time later, he was blown up by a bomb on Gallifrey, (AUDIO: Imperiatrix) but survived. This model came to an end when Alistair Gryffen's Space-Time Manipulator randomly pulled him to London in the year 2050. There, he almost immediately self-destructed to protect Gryffen and a group of teenagers from Jixen warriors. (TV: Regeneration)

The Fourth Doctor and K9 Mark II spent an indeterminate period of time together. However, the Doctor appeared to be activating K9 Mark II, implying that no significant time had elapsed since leaving Gallifrey. (TV: The Ribos Operation) It is possible that the Doctor spent some time at Nest Cottage before activating him. (AUDIO: The Stuff of Nightmares)

K9 Mark III was a gift from the Doctor to Sarah Jane Smith (TV: A Girl's Best Friend). This K9 was broken for many years, until he was repaired by the Tenth Doctor. This K9 also had a "maximum defence mode", as well as offensive capabilities, such as a nose-mounted laser gun. He sacrificed himself to save Deffry Vale High School from Krillitanes. (TV: School Reunion)
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K9 stayed with Leela on Gallifrey for many years. It was only after the disappearance of her Time Lord husband, Andred, that Leela and her K9 met Romana II and her K9. The two K9s did not get on well and competed. (AUDIO: Weapon of Choice) After Leela became Romana's bodyguard and after many important missions on her behalf, Leela's K9 was destroyed by an exploding bomb. Leela was heartbroken, but refused to have a duplicate unit built. (AUDIO: Imperiatrix)
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".
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