K9 Mark I

K9 Mark I was a portable computer originally owned by Professor Marius, who considered the robot dog his "best friend". Together, they conducted advanced medical research at the Bi-Al Foundation for an unknown length of time. When the Fourth Doctor and Leela arrived at the Foundation, Leela quickly became enamoured of the dog. The professor evnetually gave the dog to time travellers as thanks for defeating The Swarm. (DW: The Invisible Enemy)

The Mark I was particularly associated with and protective of Leela. Therefore, when she elected to remain on Gallifrey, he also chose to stay behind with her. (DW: The Invasion of Time) Nevertheless, he occasionally left Leela's side when he was sent on solo missions by the Time Lords. During these adventures he travelled in a spacecraft the Time Lords build especially for him called K-NEL. (AK9: K9 and the Missing Planet, and others)

He eventually came into the service of Lady President Romana II when Romana tapped Leela as her personal bodyguard. Together, Leela and the Mark I went on many missions together at Romana's request. He was a vital link between the two ex-companions of the Doctor, since he could easily and silently interface with Romana's K9 unit. (BFG: Square One and others)

The unit's ultimate fate was considerably unclear, since two radically different accounts existed. According to one, he was utterly destroyed in an explosion and Leela adamantly refused to have another model built in its place. (BFG: Imperiatrix). The other held that he somehow ended up on 21st century Earth, where he was forced to engage his regeneration unit and become K9 Mark 2. Thereafter he accepted a boy named Starkey as his new master, and had several adventures on Earth in the 2050s. (K9TV: Regeneration)

Later models — or marks — of K9 were based on this one.

Creation
It was unclear who, exactly, invented the K9 "concept". When he first met Leela, Professor Marius said only that he "had K9 made up" — not that he "created" or "designed" the unit – because weight restrictions did not allow him to bring his real dog from Earth into space. (DW: The Invisible Enemy)

One account claimed that K9 may have owed his existence to a temporal paradox. Far from inventing the dog himself, the professor copied the remnants of one of the versions of K9 created by the Doctor and given to Sarah Jane Smith. (BE: Tautology)

Meeting the Doctor and Leela
In the year 5000 AD, K9 aided Marius in scanning the Fourth Doctor for the Swarm inside him. He later fought off the humans infected by the Swarm on Bi-Al and on Titan Base, saving the Doctor and Leela's lives on several occasions.

The Professor offered K9 to the Fourth Doctor to travel in the TARDIS as the same weight restrictions made him unable to take Mark I back to Earth with him. (DW: The Invisible Enemy)

On the TARDIS
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
Though K9 remained on Gallifrey to "protect" Leela, he was occasionally used by the Time Lords for various solo missions. Towards this end, they gave him a specially built space vessel called K-NEL that only he could control, which gave him freedom of movement in space, but not time. (AK9: K9 and the Time Trap, et al)

K9 was with Leela on Gallifrey for many years, but it was only after the disappearance of her Time Lord husband, Andred, that she and her K9 met Romana II and her K9. Despite the fact that the two K9 units did not get on well and competed with each other, (BFG: Weapon of Choice, The Inquiry) they frequently networked together and allowed easy communication between their respective mistresses. (BFG: Square One)

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)

After being destroyed
In a way not clearly understood, K9 Mark I survived the explosion that Leela and Time Lord technology confirmed had utterly destroyed him. It is possible that Romana rebuilt him at some point, as a thank you present to Leela.

At some point after this event, and indeed after the Last Great Time War, he became an agent of the Korven. (K9TV: Hound of the Korven) During this period, he had a number of adventures whose details were not well recorded. For example, the Mark I was known to have gone to the second planet of the Montoon system and observed the Sea of Brains. (K9TV: Curse of Anubis) He also liberated the Anubians from the Huduct (K9TV: Curse of Anubis) and met Robin Hood. (K9TV: The Last Oak Tree)

Regeneration
Later, he went to the year 50,000 and met Zanthus Pia, head of the Galactic Peace Commission, whom he later saw murdered at the hands of Ahab, a Jixen emissary and his three guards. As K9 was a witness, 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 about a Korven invasion. However, they couldn't speak English and the humans believed they were attacking them. K9 came through the STM and attacked the Jixen. After a brief battle he self-destructed, destroying himself and two of the Jixen. Although working for the Korven, he cared for the safety of humans and advised them to evacuate. Using his regeneration unit, the remains transformed into a new incarnation. (K9TV: Regeneration, Hound of the Korven)

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 I returned in the first episode of K9. When asked by a fan for clarification, in light of the destruction of K9 Mark I in BFG: Imperiatrix, Bob Baker said:"Yes this is the K9 that the Doctor left with Leela on Gallifrey. In this version he was plucked out of space and time by the STM totally at random. His self-destruct in Ep 1 to save the humans results in triggering a "regeneration unit" put there presumably by the Time Lords, but of course we're not allowed to mention that."

- Bob Baker 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, or some other unknown event. No further information has become known.
 * The numbers appended to K9's "marks" have narrative origin — unlike, say, Romana I, which is actually a fan assumption. The Invasion of Time ends with the Doctor pushing a box labelled "K9 MII" into the console room, which implies that the model that stayed with Leela on Gallifrey was "K9 MI".  However, based on televised stories alone, there was no indication that this designation was programmed into the two units.  It was later established by John Leeson's dialogue in BFG: The Inquiry that K9 Mark I thought of himself as "K9 Mark I" and Romana's model as "K9 Mark II".  Nevertheless, nothing has ever clearly indicated that the Mark I was truly "the first K9 ever built, anywhere in the universe". The designation is relative to the Doctor's perception, or to relative age amongst the K9 units eventually resident on Gallifrey.