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)
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.
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)
Serving the surrounding areas of Dallas, Frisco, Allen, Richardson, Garland, & Mesquite. Since 1993, K-9 University has delivered high-class obedience training for dogs from our two serviceable locations in Garland and Plano. Since then, we have extended our services to include boarding, grooming, and doggy daycare. Our experienced and compassionate staff will train and care for your dog as if they were our own. Speak with us about our services by calling our location in Plano at (972) 801-9090 or Garland at (972) 530-7077.
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)

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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