K9 Mark IV was the fourth in the line of K9 robots. He was based upon the original K9 Mark I designed by Professor Marius in 5000, and which travelled with the Fourth Doctor and Leela. (TV: The Invisible Enemy) Sometime after she had stopped travelling with him, the Doctor had gifted K9 Mark III, an improved version of the first two K9s, to his former companion, Sarah Jane Smith. (TV: A Girl's Best Friend) K9 said that the Tenth Doctor rebuilt him with "new omniflexible hyperlink capabilities". (TV: School Reunion)