The Fourth Doctor and K9 Mark II spent an indeterminate period of time together. However, the Doctor appeared to be activating K9 Mark II, implying that no significant time had elapsed since leaving Gallifrey. (TV: The Ribos Operation) It is possible that the Doctor spent some time at Nest Cottage before activating him. (AUDIO: The Stuff of Nightmares)
During Romana's Presidency of Gallifrey, Leela became her bodyguard, and the two K9 units were forced to work together, not always amicably. After Matthias had replaced Romana as President, he nominated her to undertake a mission to find a cure for the Free Time virus, to be followed by exile. K9 opted to remain in the employ of Matthias, fighting the infection on Gallifrey. However, Braxiatel and K9 had planned another operation. Realising that the only cure to the virus was to end the regeneration capabilities of the Time Lords, Braxiatel used a Time Scoop to collect the Time Lord bio-data archive (along with K9). Romana was asked to decide whether to administer the cure or let Gallifrey die and rebuild it from scratch with the bio-data archive. (AUDIO: Gallifrey)
Whereas Sarah Jane lifted K9 Mark III into and out of her car, (TV: A Girl's Best Friend) K9 Mark IV had a "hover mode" allowing him to negotiate the stairs of the Smiths' three-story house and bi-level attic, and to enter and exit cars. (TV: The Wedding of Sarah Jane Smith, The Gift) The Eleventh Doctor implied that Mark IV was not the first to have the ability to hover. (TV: The Power of Three)
K9, occasionally written K-9, is the name of several fictional robotic canines (dogs, the name being a pun on the pronunciation of "canine") in the long-running British science fiction television series Doctor Who, first appearing in 1977. K9 has also been a central character in three of the series television spin-offs: the one-off K-9 and Company (1981), The Sarah Jane Adventures (2007–2011) and K-9 (2009–2010). Although not originally intended to be a recurring character in the series, K9 was kept in the show following his first appearance because he was expected to be popular with younger audiences. There have been at least four separate K9 units in the series, with the first two being companions of the Fourth Doctor. Voice actor John Leeson has provided the character's voice in most of his appearances, except during Season 17 of Doctor Who, in which David Brierley temporarily did so. The character was created by Bob Baker and Dave Martin, to whom rights to the character still belong; consequently, Baker's spin-off series K9, which is not BBC-produced, cannot directly reference events or characters from Doctor Who, though it attempts to be a part of that continuity.