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)

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)
K9 Mark 2 had some obvious commonalities with the Mark I that had preceded him. However, as he had almost completely lost his personal memories, he was forced to reacquaint himself with the world around him. Toward this end, he downloaded thousands of human movies onto his memdrive, which peppered his speech with 20th and 21st century English colloquialisms that his "brothers" did not typically employ. (TV: Alien Avatar) Despite his negative opinions of emotions, (TV: Mutant Copper) he asked questions about some, such as fear (TV: Fear Itself) and love (TV: Aeolian). He even showed some, laughing when he saved the day, feeling fear (TV: Fear Itself) and being angry. (TV: Regeneration)
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)

The practical challenges of working with the K9 prop have accompanied the robotic pooch in his return to Doctor Who. Producer Russell T Davies told SFX magazine, "Yes, just as we expected, multiple takes [were required] when he bumped into a door or veered off to the left. Lis Sladen did warn us, and she was right!"[7] 2009 saw two different incarnations of K9 appear regularly in two Doctor Who spin-offs: Mark IV in the third series of BBC production The Sarah Jane Adventures, and a re-vamped Mark I in the Park Entertainment production K9.
The handler and dog must also be able to pass the police test, which involves controlling the dog fully, even after giving the dog the attack command. The dog must stop immediately and fulfill any command it is given by the handler. There are specifications of training involved in using trained K9 police dogs in the airport, marine ports, railroads, in a metro station.
In early 2007, K9 was reactivated by the Tenth Doctor; Sarah Jane was unable to go to any humans for repairs prior as the parts inside K9 could change history. K9's repairs were short lived. In battling Lucas Finch and his Krillitane invasion at the school the next day, Mark III blew up a vat of Krillitane oil, which was now lethal to the Krillitanes destroying himself and the Krillitanes in the process. (TV: School Reunion)
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.[9] The article in The Times also featured a picture of the redesigned K9 for the animated series.[10] The series will be a blend of live-action and a CGI K9.
K9 Mark I, II, III, and IV addressed whoever was directing them as "Master" or "Mistress" depending upon gender, and used the formal "affirmative" and "negative" rather than "yes" and "no". They were programmed to be both loyal and logical, with a penchant for taking orders literally, almost to a fault. Their striped collars mirrored the Fourth Doctor's trademark scarf.

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The original K9 model was designed by visual effects designer Tony Harding. One early concept was to have a small actor in a large Doberman-shaped costume; however, Graham Williams vetoed this, saying that the robot should not look like a person in a costume. The eventual design was closely based on Harding's third concept sketch. (DCOM: The Invisible Enemy)
K9 dogs are expected to perform well in various situations, such as finding a suspect through smell, protecting its handler, finding hidden objects in a 150’x150′ area, knowing how to respond when under fire, and detecting explosives and narcotics. Before a handler can start working with a dog as an official Police dog, the dog must pass the normative police test, or can be bought, fully prepared, from Master Dog Training.
He usually did not engage in computer-like literal thinking, though on one occasion he followed Romana's instructions to "forget it" to the letter and erased all knowledge of tennis from his memory banks. (TV: The Stones of Blood) In another he took Romana's exclamation "Blast it!" as a command, and fired his blaster. (TV: Shada, WC: Shada, AUDIO: Shada, PROSE: Shada). Despite this, he occasionally showed a sense of humour, jokingly answering the question "What do you eat?" with "Ball bearings" (AUDIO: The Beast of Kravenos).
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)
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)

Introduced into the plot in The Invasion of Time, K9 Mark II first actually appear on-screen in The Ribos Operation (1978). More mobile than his predecessor, Mark II exhibited the ability to sense and warn others of danger. He travels as a companion alongside the Doctor and Romana. Around the time of Romana's regeneration (the transition from actress Mary Tamm to Lalla Ward), the K9 character was explained as suffering from "laryngitis" to accommodate Leeson's departure from the series at the start of the 1979–80 season; for this time, he was portrayed by David Brierley, until Leeson's return for the 1980–81 season. When the Doctor and Romana travel to the parallel universe of E-Space, K9 is severely damaged, in Warriors' Gate (1981). The damage was such that K9 could only function in E-Space; when Romana decided to stay and forge her own path, the Doctor gave K9 to her. The character makes subsequent appearances in remakes of the unfinished serial Shada, alongside the Eighth Doctor (Paul McGann); the 2003 Shada audio play and webcast depict K9 Mark II as having returned to Gallifrey with Romana, now Lady President of the Time Lords, where the Doctor visits them. It is not clear if either of these audio appearances are considered canonical in relation to the continuity of the TV series.

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