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.
In 1996, Sarah used K9 to hack into records and find Sam Jones' home address and later to translate Lost Boy's speech. (PROSE: Interference - Book One) K9 was briefly possessed by a malevolent entity, but was freed when Sarah accidentally electrocuted him. (PROSE: The Sow in Rut) K9 began slowly breaking down and eventually ended up deactivated in a cardboard box in Sarah's attic. (PROSE: Moving On) Hilda Winters had K9 dismantled and analysed. (AUDIO: Mirror, Signal, Manoeuvre)
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A short story, "Moving On", in Virgin Publishing's Decalog 3: Consequences anthology and the Big Finish Productions-produced Sarah Jane Smith audio play Mirror, Signal, Manoeuvre (both written by Peter Anghelides) indicate that K9 Mark III broke down and Sarah was unable to repair it as the replacement circuits would not be invented for several centuries.
Damaged by the time winds, K9 Mark II could not cross back into N-Space with the Doctor. He decided to stay behind at the Gateway with Romana II to help her free the remaining Tharils from slavery. (TV: Warriors' Gate) Eventually, Romana II and the presumably repaired K9 returned to their own universe and K9 Mark II came to live on Gallifrey. (PROSE: Lungbarrow, WC: Shada, AUDIO: Zagreus)
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".