All the K-9s referred to whoever owned them as "Master" or "Mistress" depending on their gender. The units were programmed to be both loyal and logical, with a penchant for taking orders literally, almost to a fault. The Fourth Doctor would often use a glib remark to disarm those who were surprised by K-9's appearance; in The Stones of Blood he said, "They're all the rage in Trenton, New Jersey." The Tenth Doctor defended its less-than-streamlined design ("...so disco!") to Rose Tyler, remarking that it was cutting edge in the year 5000.
To date, four different versions of K-9 have appeared in the series:
K-9 Mark IEdit
K-9 Mark I travelled with the Fourth Doctor and Leela from The Invisible Enemy until The Invasion of Time, when he decided to stay on Gallifrey with Leela, who had elected to remain behind on the Time Lord home world. This K-9 is the star of the 2009 K-9 television series, in which he undergoes a sort of "regeneration" process from which a new, more sophisticated and futuristic K-9 emerges. This regenerated form is "Mark 2" (a separate and distinct model from the "Mark II" below).
K-9 Mark IIEdit
After leaving K-9 Mark I with Leela, and getting back into the TARDIS, the Fourth Doctor pushed out a box with "K9 MII" written on the sides. K-9 Mark II was more mobile than his predecessor, and exhibited the ability to sense and warn others of danger. He was with the Fourth Doctor and Romana when they were shunted into the parallel universe of E-Space, and was severely damaged by time winds during the events of Warriors' Gate. The damage was such that K-9 could only function in E-Space, and when Romana decided to stay and forge her own path, the Doctor gave K-9 to her. Due to a bout of laryngitis — which the Fourth Doctor was baffled as to how the robotic dog could have contracted — the voice of K-9 Mark II changes around the time of Romana's regeneration, only to revert some time later.
K-9 Mark IIIEdit
K-9 Mark III was a gift from the Doctor to Sarah Jane Smith, and first appeared with her in the pilot episode of the aborted spin-off series K-9 and Company as well as a brief cameo appearance in the 20th anniversary ninety-minute special The Five Doctors and the revived series' episode "School Reunion". In the original outline to K-9 and Company, it would have been eventually revealed that K-9 Mark III was actually sent by and under the control of the Master, but this element never made it to the screen and was abandoned in K-9's later appearances.
K-9 was packaged in a crate by the Doctor in 1978 and sent to Sarah Jane at her aunt Lavina Smith's home in South Croydon where he waited for nearly three years in the attic before being shipped to Lavinia's new country manor with the rest of the household furnishings. A few months later, in December 1981, Sarah Jane arrived for a long-delayed visit and opened the crate, activating K-9.
By 1983, he resided with Sarah Jane in London behind a fence marked "beware of dog". K-9 Mark III's final appearance was in the second series of the new Doctor Who with David Tennant as the Tenth Doctor, alongside Elisabeth Sladen as Sarah Jane Smith in the episode "School Reunion". In that episode, K-9 appears more run-down and worn, echoing Sarah's inability to repair him in the spin-off stories (see below). Sarah Jane notes that she could not take him for repair and, one day, "he simply stopped". He prompts Mickey Smith to reevaluate his role with the Doctor and Rose Tyler, feeling himself being treated as "a tin dog". K-9 Mark III died heroically, sacrificing himself by using the last of his energy reserves to set off an explosion.
K-9 Mark IVEdit
K-9 Mark IV was a parting gift to Sarah Jane Smith from the Doctor, appearing briefly at the end of "School Reunion". He stated that the Doctor "rebuilt" him after the Mark III's sacrifice, implying that he had the same mind as the Mark III, but also confirmed he was a "brand new model", hence having a new model number. Though merchandise refers to this model as the "Mark III", close-ups of K-9's safe in the Sarah Jane Adventures lists it as the Mark IV. According to the official website for The Sarah Jane Adventures, the Doctor stored several presents for Sarah inside K-9, including a "sonic lipstick" and a watch that scans for alien life; in Enemy of the Bane, she reveals having received more than one sonic lipstick.
His systems were improved over those of his predecessor, including "omniflexible hyperlink facilities" and the ability to teleport. Additionally, whereas Sarah Jane had to lift K-9 Mark III into and out of her car, K-9 Mark IV has a hover mode which enables him to exit cars and negotiate the stairs in the Smiths' 3-storey house.
In "Invasion of the Bane", the pilot episode of The Sarah Jane Adventures, K-9 Mark IV is shown sealing off an artificially created black hole, having been at work on the project since shortly after he was given to Sarah Jane. The real-world purpose of this was to remove K-9 from subsequent episodes of The Sarah Jane Adventures, because of the concurrent development of K-9 about the regenerated K-9 Mark I.
K-9 returns from the black hole momentarily in the series 1 finale, The Lost Boy, to assist Sarah Jane against a rogue Mr Smith. He is again summoned briefly in the Doctor Who series four series final "Journey's End", linking up with Mr Smith to input the TARDIS' base codes, which would allow the Doctor to return Earth to its proper position. K-9 also appears in "From Raxacoricofallapatorius with Love", a sketch for Comic Relief featuring the cast of The Sarah Jane Adventures and comedian Ronnie Corbett.
Despite the launch of K-9 Mark I's concurrent series, K-9 Mark IV returns to the Smith household in the series 3 story The Mad Woman in the Attic, when the black hole is utilised as a powersource for a spaceship, freeing K-9 from the job. When the Trickster causes Sarah Jane to become engaged to a would-be dead solicitor, Sarah Jane fears scaring the man away, deactivates Mr Smith, and gives K-9 Mark IV to Clyde Langer. Apart from this temporary transfer, he has been with the Smith family since. Mr Smith finds K-9 annoying and, rather than inform Sarah Jane when Clyde surreptitiously borrows him to cheat on a school test, thanks the boy for taking him.