K9 Mark IV

K9 Mark IV was an intelligent robot in the form of a dog that was a companion of Sarah Jane Smith and subsequently Luke Smith, also serving as their beloved pet. He was a gift from the Tenth Doctor to Sarah Jane, the rebuilt self and replacement of K9 Mark III.

Origins
K9 Mark IV was the fourth in the line of K9 robots. He was based upon the original K9 Mark I designed by Professor Frederick 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) Before that, the Mark III had travelled with the Fourth Doctor. (PROSE: Inter-Galactic Cat)

At some point after leaving Mark the III behind, the Doctor began constructing a K9 Mark IV. The Eighth Doctor mentioned the Mark IV when he met Sarah Jane in 1996, indicating indicated that the effort in constructing the Mark III had been "nothing" in comparison to the Mark IV. (PROSE: Interference - Book Two)

The Mark III was eventually destroyed fighting off a Krillitane invasion in 2007, and so Mark IV was presented to Sarah Jane as a replacement by the Doctor, by then in his tenth incarnation. K9 explained to Sarah Jane that the Doctor "rebuilt" him with "new omniflexible hyperlink capabilities". (TV: School Reunion)

Stabilising a black hole
Sarah Jane wasn't able to enjoy Mark IV's company for long. Soon after being presented to her (approximately a year-and-a-half before Sarah Jane first met Maria Jackson), K9 Mark IV was forced to enter deep space in order to stabilise a dangerous artificial black hole in space which had been created in Switzerland. She lamented being all alone. (TV: Invasion of the Bane)

K9 was able to maintain occasional communication with Sarah Jane via a cabinet with a space-time link when the orbit of the black hole intersected with that of Earth. (TV: Invasion of the Bane) At times he was capable of returning to Earth briefly and did so on several later occasions in order to render vital assistance. He fought Mr Smith in pitched battle to allow Sarah Jane to upload the Armageddon virus. (TV: The Lost Boy) He was later called back in order to quickly upload the TARDIS' base code to Mr Smith, thereby assisting the Tenth Doctor in returning Earth to its correct location in space. (TV: Journey's End)

After each of these events, K9 went back into space to continue monitoring the black hole. Some time later, Rahnius Slitheen attempted to steal him as part of a revenge plot against Sarah Jane. K9 banished the Slitheen, after reversing one of its attacks. (TV: From Raxacoricofallapatorius with Love)

Returning to Sarah Jane
Later, Sarah Jane transferred the black hole into fuel for Eve's ship. K9 then triumphantly returned to 13 Bannerman Road to stay. He greeted Mr Smith — who held something of a resentment towards him — and had a photograph taken with Luke Smith, Clyde Langer and Rani Chandra. In 2059, Rani still had that photo. (TV: The Mad Woman in the Attic) He then helped defeat Travast Polong and the Trickster after reuniting with the Tenth Doctor. (TV: The Wedding of Sarah Jane Smith) A drawing of him by Clyde was animated by alien energy and defeated the Abomination. (TV: Mona Lisa's Revenge) He later assisted in tracking down a pair of Slitheen, locating them with his "olfactory sensors". (TV: The Gift)

Although Sarah Jane had appeared unconcerned with K9 Mark III being seen when they lived together twenty-five years earlier, she restricted Mark IV to their house except in the case of emergencies or as needed, lest she have to explain him. Luke covered by claiming K9 to be a Japanese robot toy with pre-programmed phrases. (TV: The Wedding of Sarah Jane Smith)

Moving to Oxford with Luke
When Luke Smith was trapped in a nightmare by the Nightmare Man, K9, with the help of Cention Concrete, managed to transport himself into Luke's nightmare. Soon after, K9 was temporarily damaged by the Nightmare Man. (TV: The Nightmare Man) He aided invaluably in the defeat of Ruby White by resetting the Qetesh's artificial intelligence, allowing Luke to take control of it. (TV: Goodbye, Sarah Jane Smith)

He stayed in Oxford when Luke returned home in 2011, but Luke had taken K9's whistle just in case. Luke later used the whistle to contact Mr Smith, which helped defeat John Harrison. (TV: The Man Who Never Was)

Living with Ace
After Sarah Jane's memorial, K9 was given to Ace. (WC: Farewell, Sarah Jane)

Personality
Unlike his previous model, this K9 seemed slightly more capable of having and understanding emotion. Such examples are that he said "Affection noted, mistress" after Sarah Jane called him a good dog (TV: Invasion of the Bane) and his constant rivalry with Mr Smith for the best technical support. (TV: The Mad Woman in the Attic - The Nightmare Man) Another was that he possessed a sense of right and wrong instead of simply obeying orders — for example, when Clyde tried to use K9's knowledge to help him pass a test at school. K9 tended to give long-winded explanations to why he was behaving in an unusual manner, only to have someone correctly give a simpler way of explaining it. (TV: The Gift)

Features and improvements
This K9 had a shiny metallic blue paint job and a number of improvements over Mark III including omniflexible hyperlink capabilities (TV: School Reunion) and universal serial bus ports. (TV: The Nightmare Man) Later, though, K9 reverted to his old, dark green colour. (TV: Invasion of the Bane) In his battle with Mr Smith, K9 displayed a teleport facility and utilised a laser weapon similar to the previous K9 models. (TV: The Lost Boy) Upon receiving him, Sarah Jane changed his collar colour to nearly match the pattern on the Fourth Doctor's scarf, the same or identical collar to those worn by the previous K9s. Unlike his previous models, although like K9 Mark 2, K9's eyepiece could glow at night. (TV: The Lost Boy) He also had a compartment in his side, which could hold items, such as the sonic lipsticks and the scanner watch, which were gifts from the Tenth Doctor to Sarah Jane after their last meeting. (TV: Invasion of the Bane) Whereas earlier K9s including Mark III were seen being lifted into, out of, and over things, K9 Mark IV had a hover mode, enabling him to negotiate the stairs in the Smiths' three-storey house and multi-level attic, and to access and egress automobiles on his own. (TV: The Wedding of Sarah Jane Smith, The Gift)

K9 claimed that he was shaped like a dog because "the human ego requires a familiar reference point to aid in adjusting to new concepts. Animal forms are comforting to the majority of humans." (PROSE: Housewarming)

SJA appearances
The reason for K9's fleeting appearances in The Sarah Jane Adventures was due to other parties holding the rights to the character, for use in a spin-off TV series, K9, although the character had not been completely forbidden from appearing in The Sarah Jane Adventures. As, in the first episode of the K9 series, K9 Mark I was destroyed in a self-sacrifice and "regenerated" into an entirely redesigned model and the TV series bought the rights to the concept and simply "borrowed" the character and, briefly, original design, with full consent, the rights to the character and design of the original K9 once again belong to the BBC who could use K9 (as per his original design) freely in The Sarah Jane Adventures. His minor appearances up until that time had been simply to be careful not to misuse the rights allowance whilst they were still being decided upon.

Other matters

 * The designation K9 Mark IV never actually appeared in dialogue. Documents packaged with the 1/4 scale Remote Control K9 toy referred to this model as the Mark IIIb, but props in The Sarah Jane Adventures have had the Mark IV designation.
 * K9 Mark IV is the first K9 to have been created to resurrect a previously destroyed model. This was repeated for K9 Mark I, who self-destructed and reformed into a remarkably different new body, K9 Mark 2.