K9 Mark III

K9 Mark III was the designation given to the third known version of the robot. This version was given by the Doctor as a present to his former companion, Sarah Jane Smith.

Biography
How Mark III came to be built has not been chronicled. Based upon the original K9 Mark I designed by Professor Marius (DW: The Invisible Enemy), and upon K9 Mark II, which the Doctor, in his fourth incarnation, was shown uncrating following the decision by Mark I to stay on Gallifrey (DW: The Invasion of Time), it is likely Mark III was either built by the Doctor, or obtained from the same source from which he got Mark II.

Travels with the Doctor and Adric
After the Doctor had left K9 Mark II to Romana as gift, K9 Mark III travelled with the Doctor and Adric for a short time. (DWAN: Inter-Galactic Cat, Conundrum, Planet of Paradise, Plague World, Just a Small Problem)

Travels with Sarah Jane Smith
The Doctor left K9 for Sarah Jane in 1978, but she did not learn of its existence until 1981. Soon after being uncrated, K9 helped Sarah Jane defeat the Hecate Cult of Moreton Harwood. (KAC: A Girl's Best Friend) K9 and Sarah Jane shared more adventures after this, but only a few have been chronicled. (K9 Annual 1983, et al)

About 1983, K9 Mark III still lived with Sarah Jane, but, by chance, did not accompany her to Gallifrey. (DW: The Five Doctors) Later, K9 went with Sarah to an archaeological dig in Egypt. (DWM: City of Devils) In the early 1990s, K9 assisted her in investigating a supposedly haunted house. (VD: Housewarming) In 1996, Sarah used K9 to hack into records and find Sam Jones' home address, and translate Lost Boy's speech. (EDA: Interference - Book One)

Break down
K9 was briefly possessed by a demonic pig spirit, but was freed when Sarah accidentally electrocuted him. (ST: The Sow in Rut) K9 began slowly breaking down and ended up deactivated in a cardboard box in Sarah's attic. (VD: Moving On) Later, Hilda Winters had K9 dismantled and analysed. (BFSJS: Mirror, Signal, Manoeuvre)

Sarah Jane was smart enough to undo Hilda Winters' handiwork but not enough to understand future technology. She did not show him to anyone else out of fear that his technology would rewrite human history. Sarah Jane began to carry K9's remains around in the boot of her car, having given up on being able to repair him. (DW: School Reunion)

Sacrifice
In 2007, when Sarah Jane met the Tenth Doctor, he repaired K9, but he didn't have much power. The Doctor used K9 to identify the intelligence-enhancing oil used in Deffry Vale High School's chips as Krillitane oil. During the ensuing fight with the Krillitanes, K9 bonded oddly with Mickey Smith, who identified himself as another of the Doctor's "tin dogs". When Mickey woke him to break into the school, K9 pointed out they were in a car that could be driven through the doors. When the Doctor, Rose, Sarah Jane, Mickey and Kenny were cornered, K9 rescued them and delayed the Krilitanes until he ran low on power and was left alone. When the Doctor needed to destroy the Krilitanes with their oil, K9 noted his laser could blow open the deadlocked vats, but he needed to be right next to them. When K9 fatally wounded them, their leader called K9 a "bad dog" to which he agreed before the school blew up, destroying K9 Mark III as well.

Not long after, the Doctor presented Sarah Jane with K9 Mark IV who had Mark III's memories, and the two went off for more adventures. (DW: School Reunion)

Features
Mark III included some improvements over past models. The new mechanics of K9 Mark III included larger wheels and a front and back-wheel drive. K9 was also had a brand new coat of paint (this time a metallic-blue) and a handle on the top of the shell to make carrying him easier. (KAC: A Girl's Best Friend)

Technology
K9 was an advanced computer built in the shape of a robot dog. Among the technology included were a tri-state bus driver, a universal asynchronous receiver/transmitter and holographic memory. He was powered by a self-charging nuclear battery. K9 Mark III was programmed with a wide variety of technical knowledge and scientific data which included, among other things, horticulture. K9 could also provide data printouts of any information requested from his memory banks (KAC: A Girl's Best Friend)

Behind the scenes

 * Only the pilot episode of K9 and Company, A Girl's Best Friend, was ever produced.
 * In the original outline to K9 and Company, it would have been eventually revealed that K9 Mark III was actually sent by and under the control of the Master, but this element never made it to the screen. The idea of a mechanical companion to Sarah Jane Smith being partially controlled by an enemy was recycled with Mr Smith in the first series of The Sarah Jane Adventures.
 * The Sixth Doctor make-your-own-adventure novel Search for the Doctor, published in 1986, features a descendant of Sarah Jane in the 2050s inheriting K9 Mark III. This book was published before School Reunion and the destruction of this K9.