K9 Mark IV

K9 Mark IV was an intelligent robot in the form of a dog that was a companion of Sarah Jane Smith, also serving as her beloved pet. He was a gift from the Doctor to Sarah Jane to replace his previous model; he had the memories of his predessor.

Biography
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, and which travelled with the Fourth Doctor and Leela (TV: The Invisible Enemy). In the late 1970s, 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) Mark III was eventually destroyed fighting off a Krillitane invasion, and so Mark IV was presented to Sarah Jane as a replacement by the Doctor, now in his tenth incarnation. (TV: School Reunion) The Eighth Doctor had previously mentioned a Mark IV version. (PROSE: Interference - Book Two)

The new K9 referred to himself as being rebuilt and improved, suggesting Mark IV possessed the same memory as Mark III. (TV: School Reunion) Though Sarah Jane didn't find this out right away, the Doctor had also placed some gifts in a panel in K9's side: sonic lipstick and a scanner watch that could detect alien life. They would become often-used tools in Sarah Jane's subsequent adventures. (TV: Invasion of the Bane, et al)

Sarah Jane would not be able to enjoy Mark IV's company for long. Not long after being presented to her (approximately a year before Sarah Jane first met Maria Jackson), K9 Mark IV was forced to enter deep space in order to stabilize a dangerous artificial black hole in space which had been created in Switzerland.

K9 was able to maintain occasional communication with Sarah Jane via a cabinet with a spacetime 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 to Bannerman Road 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 these events, he went back into space to continue monitor the black hole. Some time later, a member of the Slitheen family 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)

Later, Sarah Jane transferred the black hole into fuel for Eve's Ship. K9 then triumphantly returned to Bannerman Road to stay. He greeted Mr Smith — who held something of a resentment against 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 Travist 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)

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. K9 went with Luke to Oxford University. (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)

Personality
Unlike his previous model, this K9 seemed slightly more capable of having and understanding emotion. Such examples are that he did not reject Sarah Jane's affections when she was concerned for him, and his constant rivalry with Mr Smith for the best technical support. 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 an omniflexible hyperlink facility. (TV: School Reunion) 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) Later it was shown that he had a hover mode. (TV: The Wedding of Sarah Jane Smith') Also, once Sarah Jane found K9 she changed his collar colour to nearly match the pattern on the Fourth Doctor's scarf, probably to show the respect that she has for her friend. Unlike his previous models, although like K9 Mark 2, K9's eyepiece could glow at night. (TV: The Lost Boy) He also has a compartment in his side, which can hold items, such as the sonic lipstick and the scanner watch, which were gifts from the Tenth Doctor to Sarah Jane after their last meeting.

Behind the scenes

 * 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.
 * 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 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 can now use K9 (as per his original design) freely in The Sarah Jane Adventures. His minor appearances up until now have been simply to be careful not to misuse the rights allowance whilst they were still being decided upon, now that the series has begun this is no longer an issue.