K-9 Mark IV (Doctor Who Series Four)

First Appearance: Doctor Who Series Two Episode Four "School Reunion"

Played By: John Leeson

The latest model of the K-9 unit built after the previous one sacrificed itself to blow up the Krillitanes and Deffry Vale High School by the Doctor, he was a present for Sarah Jane Smith and was left in her care after he left Earth with Rose Tyler and Mickey Smith in the Tardis.

Around a year and a half before Sarah Jane adopted Luke Smith and began saving the world his aid and the aid of his friends- Maria, Clyde and (later) Rani- K-9 was forced to leave Earth and watch over an artificially created black hole (made in deep space by members of human race in Switzerland), where he has very little contact to his "mistress" or "master." Although he is still capable of visiting Earth in an emergency.


"Journey's End"

When the Doctor asks Mr. Smith to use the power from the Rift in Cardiff- sent by the Torchwood team via their Rift Manipulator- Mr. Smith informs him that he is unable to do so without having "remote access to Tardis basecode numerals." The Doctor remarks that getting the number to Mr. Smith will be a long process, however Sarah Jane tells him to let her do it. 

From the Tardis, Sarah Jane orders K-9 back to Earth, who follows his orders with his usual phrase "affirmative mistress." Luke watches as the Doctor calls him a "good dog" and orders him to transfer the basecode. Following his instructions, K-9 remarks that "the process is simple", which makes many of the "Children of Time" amused. 

K-9 stays with Luke during the planet's tow back into its rightful place in the Universe, who wraps his arms around him and cheers excitedly.