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 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".
The initial idea for realising K9 was to use a small actor inside a robotic Dobermann costume, but that was rejected in favour of a radio-controlled prop, designed by Tony Harding and made by the BBC Visual Effects Department. The Radio Control Model Centre in Harlington Middx owned by Derek Wales was commissioned to build the electronics into the first original basic shell and consequently the centre operated the dog on set for the BBC.[citation needed] The robot suffered from numerous technical problems during its time in the series, often malfunctioning because the radio controls interfered with the cameras and vice versa. On location, K9 also proved unable to traverse uneven terrain, and shots had to be conceived with this in mind. Workarounds included using a concealed piece of twine to pull the character along (this string can be clearly seen in a shot of K9 on Brighton Beach), or laying wooden planks on which it could roll.

As a Former Law Enforcement Officer, I formed Dallas K-9 with integrity. I'm the Assistant Director over the South Region for Protection Sports Association (PSA). I'm also a National level Certified Decoy for Protection Sports Association (PSA). Here at Dallas K-9 we strive to produce The Belgian Malinois, Dutch Shepherd, and German Shepherd with Solid Nerve, Extreme Drives, Clear Heads, Full crushing Grips, and strong Hunt drives with powerful noses. These qualities are needed in order to perform the job that he/she is given. Our dogs are raised around kids and are very well socialized. Our puppies are properly imprinted before being placed in their new homes to make sure it's a good fit for both the puppy and family.
Although John Leeson has been the primary voice actor for K9 since his introduction, two other actors have voiced the character. David Brierley voiced K9 during Season 17 after Leeson chose not to return to the series (Leeson subsequently signed up again for Season 18), and Roy Skelton, better known for his work voicing Daleks, voiced the character in Destiny of the Daleks when all was required of K9 was a croaking sound due to the character contracting a form of robot laryngitis.
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)
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.
When Romana eventually returned to her own universe, she brought K9 Mark II with her (presumably having repaired it) and eventually became Lady President of Gallifrey. K9 Mark II is first seen on Gallifrey in the Virgin New Adventures novel Lungbarrow by Marc Platt, alongside K9 Mark I, which had remained with Leela. Both K9 Mark I and Mark II appear, voiced by Leeson, in the Big Finish Productions audio adventure Zagreus and the Gallifrey audio series. Leela's K9 (Mark I) was destroyed at the conclusion of the second Gallifrey series and only Mark II appears in the third series.
Whereas Sarah Jane lifted K9 Mark III into and out of her car, (TV: A Girl's Best Friend) K9 Mark IV had a "hover mode" allowing him to negotiate the stairs of the Smiths' three-story house and bi-level attic, and to enter and exit cars. (TV: The Wedding of Sarah Jane Smith, The Gift) The Eleventh Doctor implied that Mark IV was not the first to have the ability to hover. (TV: The Power of Three)
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.

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)
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)

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.
K9, retroactively "K9 Mark I" (Leeson), initially appeared in The Invisible Enemy (1977) as the creation of Professor Marius (Frederick Jaeger) in the year 5000. K9 subsequently travelled with the Fourth Doctor (Tom Baker) and Leela (Louise Jameson) as a companion of the Doctor in his adventures in time and space until The Invasion of Time (1978). In this serial, K9 decides to remain on the Doctor's home planet of Gallifrey with Leela. Immediately afterwards, Doctor Who would introduce a second incarnation of K9, played by the same prop; the last scene of The Invasion of Time shows the Doctor unpacking a box labeled "K9 Mk II".
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