Although the first incarnation of K9 does not appear again in televised Doctor Who media, he is the star of the 2009 K9 television series, in which he undergoes a sort of "regeneration" process from which a new, more sophisticated and futuristic K9 emerges; in the first series of K9, the character is transported to London c. 2050 by Professor Gryffen (Robert Moloney). Though on regeneration the character loses his memory of his adventures with the Doctor, he assists Gryffen and several teenage companions against a dystopian regime of "The Department". The upgraded K9 has new specifications, sporting a sophisticated head-up display, the ability to fly, and more powerful laser weapons.
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)
In 1985, a series of gamebooks featuring the Sixth Doctor were published by Severn House under the title Make your own adventure with Doctor Who in Britain and Find Your Fate — Doctor Who in the United States. These books were actually written by scriptwriters for the television series. Martin wrote Search for the Doctor which takes place in the mid-21st century and features K9 Mark IV being reunited with the Sixth Doctor long after Sarah's death.
After K9 Mark I regenerated into K9 Mark 2, his design was radically altered. His ear probes were replaced with larger silver aerials. A set of blue lights on his head flashed when he spoke. Most of his body was dark silver and a silver dog bone was located under his neck. He seemed to have the same capabilities as Mark I but was presumably more advanced. (TV: Regeneration)
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)
Later he defeated Ahab, who tried to convince him he had murdered Zanthus Pia. He became part of the K9 Unit, being made the responsibility of Gryffen and under the control of Department inspector June Turner. (TV: The Bounty Hunter) He foiled Inspector Drake's plan to control the population using Cerulium (TV: Sirens of Ceres) and experienced fear. (TV: Fear Itself) He aided in the defeat of echoes of Gryffen's family, (TV: The Fall of the House of Gryffen) Orthrus, a duplicate of him created by Drake to frame him (TV: Jaws of Orthrus) and the Bodach. (TV: Dream-Eaters) He met Nehetka and Geb of the Anubians, a race his previous incarnation liberated. He was so obsessed with regaining his lost memories that he didn't notice Gryffen, Starkey and Jorjie's takeover by the Anubians. He soon found out when the Anubians attacked the Department. With Darius' help, he defeated them. (TV: Curse of Anubis)
Having left K9 Mark III to Sarah Jane (TV: A Girl's Best Friend) following their travels with Adric in his fourth incarnation, (PROSE: Just a Small Problem) the Doctor eventually built K9 Mark IV. The Eighth Doctor mentioned Mark IV when he met Sarah Jane in 1996. He indicated that the effort in constructing Mark III was "nothing" in comparison to Mark IV. (PROSE: Interference - Book Two)
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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.