K9 Mark I

K9 Mark I is the main tritagonist of Season 15 of Doctor Who.
He was voiced by John Leeson, who has reprised the role for Big Finish.
==Biography==
K9 was created by Professor Marius, a specialist in extra-terrestrial medicine, and served as his assistant at the Bi-Al Foundation hospital asteroid. He encountered the Doctor and Leela when they came to the asteroid after the Doctor became the host for the Nucleus of the Virus of the Purpose, a sentient virus. K9 worked with Leela to defend Marius' laboratory from staffmembers controlled by the Virus but was briefly infected himself, which allowed the Virus' servants to retrieve the Nucleus. After helping cure everyone infected at the foundation, K9 travelled with the Doctor and Leela to Titan, where he helped them destroy the Nucleus' breeding chambers and killed its main servant, Supervisor Lowe. Afterwards, with Marius due to return to Earth, he asked the Doctor and Leela to take K9 with them in the TARDIS.
After taking a bit of time out while the Doctor carried out upgrades, in The Sun Makers, K9 joined them in their exploration of Pluto in the far future, where humanity was enslaved by the Usurians. K9 helped a rebellion overthrow the Usurians' Company and save Leela from being steamed to death. In Underworld, he later joined them in an encounter with a Minyan spacecraft, searching for their race bank, the P7E. K9 helped to power and guide the Minyans' ship, making their mission a success.
K9, in The Invasion of Time, aided the Doctor in his plan to appear to assist a Vardan invasion of Gallifrey while seeking to stop it. K9's non-organic mind was able to interface with the Gallifreyan Matrix, allowing him to place the Vardans in a time loop. It was only then that they learned the Vardans had simply been a front for the Sontarans, who also invaded. K9 was able to direct Gallifreyan technician Rodan to construct the De-Mat Gun which the Doctor used to defeat the Sontarans. Afterwards, Leela elected to remain on Gallifrey with Commander Andred, and K9 chose to remain with her.
The Gallifrey audio plays had K9 operating on Gallifrey and shared a fraught relationship with his equivalent, K9 Mark II. K9 Mark I was eventually destroyed by an explosion. He is the version used by K9's creators in Doctorless adventures, notably a series of children's books and an Australian children's series.
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