The Fourth Doctor and K9 Mark II spent an indeterminate period of time together. However, the Doctor appeared to be activating K9 Mark II, implying that no significant time had elapsed since leaving Gallifrey. (TV: The Ribos Operation) It is possible that the Doctor spent some time at Nest Cottage before activating him. (AUDIO: The Stuff of Nightmares)
Either Inspector Thorne or Lomax found out about K9's regeneration unit and decided to acquire it. Thorne joined forces with Freddie Maxwell, owner of Crashclub involving old robots being made to fight. K9 went undercover as a gladiator and made friends with the robot clowns Chuckles and Boris. He was called to fight the Pain-Maker, who was laced with the explosive solarmite. When he was set to self-destruct, K9 detected this and fled the explosion. (TV: Robot Gladiators) To regain his memory and find out his connection to the STM, K9's archive was scrambled. Directives from the STM almost caused him to self-destruct and lose all his memory. However, with Gryffen and Starkey's help he overrode the directives. (TV: Mind Snap) He helped Gryffen find the STM's temporal stabiliser from the fallen angel crash site in Canada, where it was revealed to be Korven technology. (TV: Angel of the North) He stopped haywire CCPCs when The Last Precinct's plan backfired. (TV: The Last Precinct)
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.
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)
Within the series' narrative, K9 is a robot dog acquired by Doctor Who's title character in the 1977 serial The Invisible Enemy. The first two incarnations of the character travelled alongside the Fourth Doctor (portrayed by Tom Baker) until 1981. In these stories, K9 proved useful for the powerful laser weapon concealed in his nose, his encyclopaedic knowledge and his vast computer intelligence. By 1981, each of the two models of K9 which travelled alongside the Doctor had been left with one of the Doctor's female companions. The character subsequently transitioned into spin-off territory. Producers hoped K9's popularity with children would launch the series K9 and Company, led by actress Elisabeth Sladen in the role of companion Sarah Jane Smith alongside a new K9. In 2006, K9 reappeared in the revived series of Doctor Who. Although the character's appearances in the 2007 spin-off The Sarah Jane Adventures were hampered by the concurrent development of the K9 spin-off, the character began to appear full-time in both K9 and The Sarah Jane Adventures, depicting adventures of the original K9 Mark I and of Mark IV, respectively.
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