K9 stayed with Leela on Gallifrey for many years. It was only after the disappearance of her Time Lord husband, Andred, that Leela and her K9 met Romana II and her K9. The two K9s did not get on well and competed. (AUDIO: Weapon of Choice) After Leela became Romana's bodyguard and after many important missions on her behalf, Leela's K9 was destroyed by an exploding bomb. Leela was heartbroken, but refused to have a duplicate unit built. (AUDIO: Imperiatrix)
A third model, K9 Mark III was evidently constructed by the Fourth Doctor; indeed, the Eighth Doctor did not dispute the presumption that he had built this K9. (PROSE: Interference - Book Tqo) Mark III travelled with the Fourth Doctor and Adric for a time. (PROSE: Inter-Galactic Cat) Eventually, however, he was shipped in 1978 to Sarah Jane Smith at the home she had shared with her Aunt Lavinia in South Croydon during her companionship of the Third and Fourth Doctors. (TV: A Girl's Best Friend) Since parting ways with the Doctor (TV: The Hand of Fear) however, Sarah Jane had already gone away and was travelling extensively. The following year, Lavinia purchased a manor house and market garden in Moreton Harwood, Gloucester, and took the still crated K9 with her. On 18 December 1981, Sarah Jane and her foster brother Brendan Richards arrived at Lavinia's home for Christmas, but Lavinia had left early for her lecture series in the United States. Sarah Jane opened the crate and received her new companion. (TV: A Girl's Best Friend)
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.
This K9 exhibited the ability to sense and warn others of danger. He was also more mobile than his predecessor, despite his problems with the swamps of Delta III, able to lift himself up on small support rods so that he could climb stairs (PROSE: The Shadow of Weng-Chiang, TV: The Power of Kroll, The Armageddon Factor) The Doctor often used a glib remark to disarm those who were surprised by K9's appearance, such as "They're all the rage in Trenton, New Jersey." (TV: The Stones of Blood)
While caught up in a Time war between the past and the future on the planet Aoris, K9 was forced to spend a thousand years hidden on the planet, setting himself up as 'the great Machina' so that he could found a cult that would aid the Doctor and Romana in bringing the war to an end, hiding behind a fake computer with a version of the TARDIS until he could return to the Doctor and Romana once the war ended (AUDIO: The Paradox Planet, Legacy of Death).
To run the program, you must register on the Web site to get the free activation code. The control panel is accessible only through the Internet. Uninstalling requires removing the app from the Task Manager, then using its password to delete it from the Windows Control Panel, and finally, rebooting. Because the password gets sent to the e-mail account of the person who registered the program, it's possible that an enterprising user could disable K9 on a shared computer.
K9 Mark IV was a recreation and enhancement of the Mark III created by the Tenth Doctor as a second present for Sarah Jane. (TV: School Reunion) This model continued in service to his mistress, Luke Smith, Clyde Langer, and Rani Chandra and was known by Maria Jackson and Kelsey Hooper. He was left fixing a black hole in Sweden for many years, but after returning to Earth, was used sparingly, until he went to university with Luke Smith. (TV: The Nightmare Man)
The Eighth Doctor Adventures novel Interference: Book Two by Lawrence Miles indicates that the Doctor built a Mark IV model sometime prior to the events of that book, but what happened to this unit is not stated. In the novel The Gallifrey Chronicles by Lance Parkin, K9 Mark II makes another appearance, having been trapped within the TARDIS since the events of The Ancestor Cell. At the end of The Gallifrey Chronicles the Doctor sends K9 on a secret mission to Espero, presumably to seek out his former companion, the living TARDIS known as Compassion.
Jealous, Possessive by Paul Magrs, the "Scorpio" story in Big Finish's anthology Short Trips: Zodiac features K9 Mark I and Mark II relaying their exploits to each other, and their veiled put-downs to each other reveal that each considers the other to be the "inferior" version. This attitude is also occasionally hinted at in the way the two units refer to each other in the Gallifrey audio series.