From TARDIS Index File, the free Doctor Who reference.

"Ready, master."

―K-9 to the Doctor

K-9 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 K-9.

History

On the TARDIS

Professor Marius invented the first K-9 because weight requirements did not allow him to bring his real dog from Earth into space (DW: The Invisible Enemy) though K-9 may owe his existence to a temporal paradox. According to one account, the professor copied the remnants of one of the version of K-9 created by the Doctor and given to Sarah Jane Smith. (DWM: Tautology)

The Professor offered K-9 to the 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).

K-9 playing chess against the Fourth Doctor. (DW: The Sun Makers)

On Gallifrey

There are (possibly apocryphal) accounts that this model of K-9 later performed missions on behalf of the Time Lords and was given control of a space vessel called K-NEL (The Adventures of K9: K9 and the Time Trap, et al).

On Earth

K-9 Mark I is known to have survived the Last Great Time War. At some point while tracking members of a race known as the Jixen he was pulled to mid-21st century Earth. Upon arrival he self-destructed both destroying himself and the Jixen. Unlike other K-9 models, Mark I had by this time been equipped with a regeneration circuit which allowed him to be recreated as a new model. (K9TV: Regeneration) However, it is possible that Mark I escaped when Gallifrey was transported next to Earth. (DW: The End of Time)

Features

The mechanics of K-9 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)

The Doctor and K9 are engaged in a pleasant game of chess.(DW: The Sun Makers)

Behind the Scenes

• K9 Mark 1 is rumoured to return in the first episode of K-9 (TV series). Bob Baker has said that the K9 model that appears in the early part of the first episode is "identical to the one Leela had in Galifrey [sic]". This means K9 Mark 1 might have survived the Time War. In the pilot episode, he will appear as last seen in Doctor Who but will before expiring, insist his companion initiate his "regeneration" sequence, from which a new and more futuristic form of K-9 Mark I will emerge.
• Although another actor's voice is heard as K-9 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.