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.

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)
In 1990, an unspecified K9 unit appeared with Sylvester McCoy as the Seventh Doctor and Sophie Aldred as Ace in an episode of the children's education programme Search Out Science entitled Search Out Space, which was included as an extra on the release of the Doctor Who story Survival by the BBC. Another unspecified K9 unit also appeared in the 1993 charity special Dimensions in Time. In the 1998 computer game Fallout 2, the Navarro base has a damaged robot dog known as K9, which uses similar speech mannerisms to the Infobox Doctor Who character. If repaired, the cyberdog is willing to join the character's party as a companion. In the 1999 television series Queer as Folk (written by future Doctor Who executive producer Russell T Davies), a K9 model is given to the character Vince as a birthday present. The prop used was an original, operated — as occasionally in Doctor Who — by visual effects assistant Mat Irvine. In the second series of I'm Alan Partridge (2002), the character of Alan Partridge recalls how his purchase of the rights to K9 contributed to his mental breakdown and driving to Dundee in his bare feet while gorging on Toblerone. In the South Park episode "Go God Go XII" (2006), Eric Cartman, being trapped in the year 2546, has acquired a robot dog called "K-10", a parody of K9. Due to timeline alterations, he is replaced by robot cat "Kit-9" and later robot bird, "Cocka-3". K9 appeared on a special Doctor Who-themed edition of The Weakest Link in 2007, but was voted out unanimously at the end of the first round, despite answering his question correctly. This was due to the fact every single player of the team answered correctly and banked the £5,000 target, and that the producers told the contestants to vote him off, just in case he broke down. Anne Robinson (whom K9 addressed as "Mistress") said "I'm so sorry" before declaring him the weakest link.[citation needed]
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)
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)
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 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.

K9, occasionally written K-9, is the name of several fictional robotic canines (dogs, the name being a pun on the pronunciation of "canine") in the long-running British science fiction television series Doctor Who, first appearing in 1977. K9 has also been a central character in three of the series television spin-offs: the one-off K-9 and Company (1981), The Sarah Jane Adventures (2007–2011) and K-9 (2009–2010). Although not originally intended to be a recurring character in the series, K9 was kept in the show following his first appearance because he was expected to be popular with younger audiences. There have been at least four separate K9 units in the series, with the first two being companions of the Fourth Doctor. Voice actor John Leeson has provided the character's voice in most of his appearances, except during Season 17 of Doctor Who, in which David Brierley temporarily did so. The character was created by Bob Baker and Dave Martin, to whom rights to the character still belong; consequently, Baker's spin-off series K9, which is not BBC-produced, cannot directly reference events or characters from Doctor Who, though it attempts to be a part of that continuity.
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