K9 Mark III

"K-9! Rose Tyler, Mickey Smith, allow me to introduce K-9! Well, K-9 Mark III, to be precise."

- The Tenth Doctor introducing K-9 to Mickey and Rose

K-9 Mark III was presumably built at some point by the Doctor, which he left on Earth as a present for Sarah Jane Smith.

Travels with Sarah Jane Smith
K-9 assisted Sarah Jane in defeating the Hecate worshippers of Moreton Harwood. (KAC A Girl's Best Friend)Circa 1983, K-9 Mark III still lived with Sarah Jane, but, by chance, did not accompany her to Gallifrey. (DW: The Five Doctors) K-9 accompanied Sarah to an archaeological dig in Egypt. (DWM: City of Devils) In the early 1990s, K-9 assisted Sarah in investigating a supposedly haunted house. (VD: Housewarming) In 1996, Sarah used K-9 to hack into records and find Sam Jones' home address, and later to translate Lost Boy's speech. (EDA: Interference - Book One)

Break down
K-9 was briefly possessed by a malevolent entity, but was freed when Sarah accidentally electrocuted him. (ST: The Sow in Rut) K-9 began slowly breaking down and eventually ended up deactivated in a cardboard box in Sarah's attic. (VD: Moving On)

Hilda Winters had K-9 dismantled and analysed. (BFSJS: Mirror, Signal, Manoeuvre)

Sacrifice
In 2007, when Sarah Jane met the Doctor again, he successfully repaired K-9 Mark III, but K-9 sacrificed itself to stop the Krillitanes. He was once again rebuilt by the Doctor before his departure and K-9 joined Sarah Jane once again as her companion. (DW: School Reunion)

Features
When the Doctor left Sarah Jane Smith on Earth, he created K-9 Mark III to be Sarah's new companion. The new mechanics of K-9 Mark III included larger wheels in conjunction with a front and back-wheel drive. (K-9 was given a brand new coat of paint (this time a metallic-blue) plus a handle on the top of the shell to make carrying him easier. (KAC: A Girl's Best Friend)

Behind the Scenes

 * Only the pilot episode of K-9 & Company, A Girl's Best Friend, was ever aired.
 * The original outline to K-9 and Company, it would have been eventually revealed that K-9 Mark III was actually sent by and under the control of the Master, but this element never made it to the screen.