When being imprisoned by the Sporraxx they tried to access K9's memory core. Memories that K9 had assumed to be erased were slowly coming back and the Sporraxx got to know more and more about K9's past. They reprogrammed K9 to protect a small spherical marauder bomb, that would destroy Earth. However K9 was able to break free of their control, because deep in his memories a sense of right and wrong prevailed. He took the bomb back into the Sporraxx space ship where it exploded. (COMIC: Short Circuit)
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After debuting in the final scene of "School Reunion" (2006), K9 Mark IV returned in spin-off series The Sarah Jane Adventures' premiere episode "Invasion of the Bane", where due to licensing issues with creator Bob Baker it is explained that K9 is sealing a black hole and can only communicate briefly and infrequently with Sarah Jane. The character makes an heroic appearance in the first series finale The Lost Boy, to do battle with rogue alien supercomputer Mr Smith (Alexander Armstrong), demonstrating new CGI teleportation and hovering features. K9 Mark IV's interfaces include at least one USB port. Subsequently, the character appears briefly in the Doctor Who series four series finale "Journey's End" (2008), wherein K9 and Mr Smith assist the Doctor in returning Earth to its proper position. K9 next appears in the comedic Red Nose Day 2009 Sarah Jane mini-episode "From Raxacoricofallapatorius with Love". A deal with the creators of K9 having been struck, K9 Mark IV became a regular character in The Sarah Jane Adventures in third series story, The Mad Woman in the Attic (2009), until the series four premiere The Nightmare Man (2010), where he accompanies departing series regular, Sarah Jane's adoptive son Luke Smith (Tommy Knight), to university. He appears again in series four finale, Goodbye, Sarah Jane Smith. Though he doesn't appear in the serial, it is mentioned in The Man Who Never Was (2011) that Luke invented a special dog whistle to summon him with.
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's innards were redesigned twice more over the course of the series, firstly in collaboration with a company called Slough Radio Control. It allowed one of its employees, Nigel Brackley, to be seconded to the series semi-permanently to supervise the prop. Brackley, who has since gone on to a career in the movie industry, controlled K9 for many of its studio appearances. Eventually, there came a point where the dog's inherent liabilities were outweighing his assets, and the internal mechanisms were completely rebuilt by designer Charlie Lumm. The wheels were enlarged and given independent drives for power and better manoeuvrability, and the radio controls were switched from AM to FM signals to resist interference. However, by the time the improved model made his debut in State of Decay, the first story recorded for Season 18, the decision had already been made to write the character out of the series in the adventure Warriors' Gate.
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.