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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In 1996, Sarah used K9 to hack into records and find Sam Jones' home address and later to translate Lost Boy's speech. (PROSE: Interference - Book One) K9 was briefly possessed by a malevolent entity, but was freed when Sarah accidentally electrocuted him. (PROSE: The Sow in Rut) K9 began slowly breaking down and eventually ended up deactivated in a cardboard box in Sarah's attic. (PROSE: Moving On) Hilda Winters had K9 dismantled and analysed. (AUDIO: Mirror, Signal, Manoeuvre)
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)
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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)
K9's innards were redesigned twice more over the course of the series, firstly in collaboration with a company called Slough Radio Control. It allowed one of its employees, Nigel Brackley, to be seconded to the series semi-permanently to supervise the prop. Brackley, who has since gone on to a career in the movie industry, controlled K9 for many of its studio appearances. Eventually, there came a point where the dog's inherent liabilities were outweighing his assets, and the internal mechanisms were completely rebuilt by designer Charlie Lumm. The wheels were enlarged and given independent drives for power and better manoeuvrability, and the radio controls were switched from AM to FM signals to resist interference. However, by the time the improved model made his debut in State of Decay, the first story recorded for Season 18, the decision had already been made to write the character out of the series in the adventure Warriors' Gate.
Damaged by the time winds, K9 Mark II could not cross back into N-Space with the Doctor. He decided to stay behind at the Gateway with Romana II to help her free the remaining Tharils from slavery. (TV: Warriors' Gate) Eventually, Romana II and the presumably repaired K9 returned to their own universe and K9 Mark II came to live on Gallifrey. (PROSE: Lungbarrow, WC: Shada, AUDIO: Zagreus)
On 3 April 2010, the TV series K9 began broadcasting on the Australian Television Network Ten as part of a Saturday morning line-up of children's shows. John Leeson is the voice of K9. The first episode called "Regeneration"[13] featured the earlier Mark I version of K9. The "older" K9 materializes in London in the future and in the midst of an incident where an alien species is threatening human lives. K9 sacrifices himself in order to save the humans, but one component survives that enables his self-regeneration into the new unit. The main human cast members include Robert Moloney as Professor Gryffen, Keegan Joyce as Starkey (K9's new "master"), and Daniel Webber as Darius. The first 14 episodes of K9 season 1 were broadcast mid-morning on Channel 5 in December 2010 with the remaining episodes scheduled for January 2011. Trailers for the series appeared on Channel 5 from early December voiced by John Leeson. Originally, it had been planned to split the series in two halves between the Christmas 2010 and Easter 2011 school holiday schedules.
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).
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)
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)
During an encounter with the sentient painting, the Mona Lisa at the International Gallery, a simulation of K9 Mk IV was created when Mona Lisa was tricked into animating a drawing of K9 made by Clyde inside a cloud of mystic drawing-animation energy. He subsequently defeated The Abomination, another evil painting-come-to-life, before disappearing as the Mona Lisa herself reverted to being an immobile painting. (TV: Mona Lisa's Revenge)

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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On August 1, 2016, Blue Coat, Inc. (K9’s parent company) was acquired by cybersecurity leader Symantec™.  As can be imagined Blue Coat and Symantec had a handful of similar products and unfortunately, it didn’t make sense to maintain two competing products. it was decided to “end-of-life” K9 Web Protection and focus corporate and customer attention on Symantec’s line of Norton™ products.
After this he was tricked by Thorne to give him his regeneration unit for a lost memory chip. This turned him into a bomb to make him self-destruct on encountering Jixey. Starkey helped him disarm himself. (TV: Hound of the Korven) K9 Mark 2's brief life was almost cut tragically short when after the fall of the Korven invasion force, Thorne sent the Trojan to kill K9, who defeated it by burning out his power core and exhausting all his energy. As his regeneration unit was in the Trojan, K9 seemingly died at the mansion after his companions bade him farewell. However his regeneration unit homed in at the last minute and resurrected him, giving the frisky pup a new lease on life. The regeneration unit restored his old dog collar so he could celebrate the joyful occasion to the full as he flew off to more adventures, bristling with life. (TV: The Eclipse of the Korven)

After debuting in the final scene of "School Reunion" (2006), K9 Mark IV returned in spin-off series The Sarah Jane Adventures' premiere episode "Invasion of the Bane", where due to licensing issues with creator Bob Baker it is explained that K9 is sealing a black hole and can only communicate briefly and infrequently with Sarah Jane. The character makes an heroic appearance in the first series finale The Lost Boy, to do battle with rogue alien supercomputer Mr Smith (Alexander Armstrong), demonstrating new CGI teleportation and hovering features. K9 Mark IV's interfaces include at least one USB port.[2] Subsequently, the character appears briefly in the Doctor Who series four series finale "Journey's End" (2008), wherein K9 and Mr Smith assist the Doctor in returning Earth to its proper position. K9 next appears in the comedic Red Nose Day 2009 Sarah Jane mini-episode "From Raxacoricofallapatorius with Love".[3] A deal with the creators of K9 having been struck, K9 Mark IV became a regular character in The Sarah Jane Adventures in third series story, The Mad Woman in the Attic (2009), until the series four premiere The Nightmare Man (2010), where he accompanies departing series regular, Sarah Jane's adoptive son Luke Smith (Tommy Knight), to university. He appears again in series four finale, Goodbye, Sarah Jane Smith. Though he doesn't appear in the serial, it is mentioned in The Man Who Never Was (2011) that Luke invented a special dog whistle to summon him with.

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