K9 Mark I

K9 Mark I was built by Professor Marius to accompany and assist him in his work at the Bi-Al Foundation. He served as the template for all future versions of K9.

When the Fourth Doctor and Leela arrived at the Foundation they took a liking to K9 and he was gifted to them by the Professor. From then on, the Doctor saw the robot as a good friend.

On the TARDIS
Professor Marius invented the first K9 because weight requirements did not allow him to bring his real dog from Earth into space (DW: The Invisible Enemy) though K9 may owe his existence to a temporal paradox. According to one account, the professor copied the remnants of one of the version of K9 created by the Doctor and given to Sarah Jane Smith. (DWM: Tautology)

The Professor offered K9 to the Fourth Doctor as the same weight requirements made him unable to take Mark I back to Earth with him. (DW: The Invisible Enemy) After a number of adventures with Leela and the Doctor, Mark I decided to stay on Gallifrey with his "mistress". (DW: The Invasion of Time).

On Gallifrey
There are (possibly apocryphal) accounts that this model of K9 later performed missions on behalf of the Time Lords and was given control of a space vessel called K-NEL. (AK9: K9 and the Time Trap, et al)

K9 accompanied Leela on Gallifrey for many years, but it was only after the disappearance of her Time Lord husband, Andred, that Leela and her K9 met Romana and her K9 (the two K9 units did not get on well and competed with each other). (BFG: Weapon of Choice)

After Leela became Romana's bodyguard, and after many important missions on her behalf, Leela's K9 was destroyed in a bomb explosion, leaving Leela heartbroken, but refusing to allow a duplicate unit to be built in its place. (BFG: Imperiatrix)

Later Life
K9 Mark I survived the Last Great Time War, despite having been destroyed earlier. This was probably due to Romana rebuilding him, as it is likely that she put his regeneration circuits in after she rebuilt him. At some point he somehow became an agent of the Korven, possibly on the orders of Romana and Leela to make sure that he survived the time war. (K9TV: Hound of the Korven)

Later he went to the year 50,000 and met Zanthus Pia, head of the Galactic Peace Commission who he saw murdered at the hands of Ahab, a Jixen emissary and his three guards. Being a witness to the murder the Jixen attacked K9 (K9TV: The Bounty Hunter). The Jixen were pulled to 2050 London via a Space-Time Manipulator and tried to warn the humans in the area about a Korven invasion. However, they couldn't speak English and the humans believed they were attacking them. K9 then came through the STM and attacked the Jixen, after a brief battle he self-destructed to kill the emissary, both destroying himself and the Jixen (bar two). Although working for the Korven he still seemed to care for the safety of humans and advised them to evacuate. Unlike other K9 models, Mark I had by this time been equipped with a regeneration unit which allowed him to be recreated as a new model - the regeneration circuit was probably installed by Romana after she possibly rebuilt him. (K9TV: Regeneration, Hound of the Korven)

Undated events

 * Travels to the second planet of the Montoon system and observed the Sea of Brains. (K9TV: Curse of Anubis)
 * Liberates the Anubians from the Huduct. (K9TV: Curse of Anubis)
 * Meets Robin Hood. (K9TV: The Last Oak Tree)

''These events could have occurred between the end of the Last Great Time War and his regeneration or on his missions for the Time Lords. They could not have occurred in his travels with the Doctor, as K9 doesn't remember the Doctor, so he would not have memory of these events either.''

Personality
K9 was often left behind in the TARDIS and so he saw journeying out as a treat. (DW: The Sun Makers) He stated that to wish was to have emotion and so he wished for nothing. Despite this he regularly showed emotion, even sulking when shouted at. The Doctor described him as overbearing, arrogant and insufferable, after K9 told him there was only a 1% chance of the Doctor having a better explanation for something than him. (DW: The Invasion of Time)

Features
The mechanics of K9 Mark I included rotating ear-probes, telescopic 'eye' probe, extendable nose, flashing lights on the top and the 'eye' panel, waggling tail antennae, and ticker-tape tongue. The head could also move up and down. The shell was painted in gold/grey metallic. Around K9's neck was a tartan collar, from which hung a silver disk. (DW: The Invisible Enemy) By the time of his liberation of the Anubians, K9 was modified (likely by the Time Lords or the Korven) so he could fly through the vacuum of space. (K9TV: Curse of Anubis) K9 had a vast database inherited by his next incarnation containing data on the following aliens:
 * Jixen
 * Meron
 * Mermaids of Gorgol XIII
 * Korven
 * Sporax
 * Oroborus
 * Medes
 * Aeolian

Aside from the Korven or Jixen, it is unknown whether he actually encountered any of these species before he encountered them as K9 Mark 2.

Behind the scenes

 * K9 Mark 1 returned in the first episode of K9. Bob Baker had said that the K9 model that appears in the early part of the first episode is "identical to the one Leela had on Gallifrey". This could be a result of an alteration of known history resulting in K9's removal from Gallifrey sometime prior to his destruction portrayed in Imperiatrix, K9 being rebuilt after his destruction and living through the Time War on Gallifrey, this K9 Mark 1 being an identical copy of the destroyed K9, or some other unknown event. No further information has become known.
 * Although another actor's voice was heard as K9 in the new series' promotional trailer, it was later confirmed that John Leeson would once again perform the character's choice for the TV series.