K9

K9 — occasionally written as K-9 or K•9 — was the designation given to a series of intelligent, dog-like robots who served as companions of Professor Marius, the Fourth Doctor, Leela, Romana, the Mistress, Sarah Jane Smith, Luke Smith, Starkey, and the Tenth Doctor.

K9 Mark I, II, III, and IV addressed whoever was directing them as "Master" or "Mistress" depending upon gender, and used the formal "affirmative" and "negative" rather than "yes" and "no". They were programmed to be both loyal and logical, with a penchant for taking orders literally, almost to a fault. Their striped collars mirrored the Fourth Doctor's trademark scarf.

Professor Frederick Marius, who invented the first K9 in the year 5000 while working on the asteroid K4067, described him as "my best friend and constant companion." Marius had a dog on Earth, but weight requirements did not allow him to bring his real dog into space, so he built K9. (TV: The Invisible Enemy) He built K9 by using the latest technology. Marius used his own medical computer, a state-of-the-art intraresponsive brain app, protective anti-radiation cladding, probes, a laser scapel, a vision and voice box and two scanning antennas. He put these parts together and created K9. (PROSE: One Man and His Dog) According to one account, the Professor copied the remnants of one of the versions of K9 created by the Doctor and given to Sarah Jane Smith. (PROSE: Tautology)

The Professor offered K9 to the Fourth Doctor as the same weight requirements made him unable to take Mark I back to Earth. (TV: The Invisible Enemy) After adventures with Leela and the Doctor, Mark I decided to stay on Gallifrey with his "mistress". (TV: The Invasion of Time).

The Fourth Doctor and K9 Mark II spent an indeterminate period of time together. However, the Doctor appeared to be activating K9 Mark II, implying that no significant time had elapsed since leaving Gallifrey. (TV: The Ribos Operation) It is possible that the Doctor spent some time at Nest Cottage before activating him. (AUDIO: The Stuff of Nightmares)

There are accounts that this model of K9 went on missions on behalf of the Time Lords and was given control of a space vessel called K-NEL. (K9 and the Time Trap, et al)

K9 stayed with Leela on Gallifrey for many years. It was only after the disappearance of her Time Lord husband, Andred, that Leela and her K9 met Romana II and her K9. The two K9s did not get on well and competed. (AUDIO: Weapon of Choice) After Leela became Romana's bodyguard and after many important missions on her behalf, Leela's K9 was destroyed by an exploding bomb. Leela was heartbroken, but refused to have a duplicate unit built. (AUDIO: Imperiatrix)

However, K9 survived this and the Last Great Time War. He witnessed the murder of Zanthus Pia by Ahab. His Jixen henchmen fought him and were transported to 2050 Earth. K9 self-destructed to protect the humans nearby, then regenerated into a more advanced model. (TV: Regeneration, The Bounty Hunter)