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".
During Romana's Presidency of Gallifrey, Leela became her bodyguard, and the two K9 units were forced to work together, not always amicably. After Matthias had replaced Romana as President, he nominated her to undertake a mission to find a cure for the Free Time virus, to be followed by exile. K9 opted to remain in the employ of Matthias, fighting the infection on Gallifrey. However, Braxiatel and K9 had planned another operation. Realising that the only cure to the virus was to end the regeneration capabilities of the Time Lords, Braxiatel used a Time Scoop to collect the Time Lord bio-data archive (along with K9). Romana was asked to decide whether to administer the cure or let Gallifrey die and rebuild it from scratch with the bio-data archive. (AUDIO: Gallifrey)
During training at Master Dog Training, a K9 officer will better understand his or her dog, and the police dog will flawlessly perform its duties. During work and practice, the handler will be able to improve his or her dog’s performance, rather than ruin it by doing something incorrectly. For a K9 Police officer, we will solve any behavior problems that the dog might be displaying, and will teach the handler how to correct these problems during work. During the K9 police test, we will be teaching how to properly pass it through all the different disciplines.
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 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 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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