K9 Mark IV

K-9 Mark IV was an intelligent robot in the form of a dog that was a companion of Sarah Jane Smith.

Biography
K-9 Mark IV is the fourth in the line of K-9 robots, and was based upon the original K-9 Mark I designed by Professor Marius and which travelled with the Doctor and Leela (DW: The Invisible Enemy). In the late 1970s, the Doctor had gifted K-9 Mark III, an improved version of the first two K-9s, to his former companion, Sarah Jane Smith. (KAC: A Girl's Best Friend) Mark III was eventually destroyed fighting off a Krillitane invasion, and so Mark IV was presented by the Doctor, now in his tenth incarnation to Sarah Jane as a replacement. It is not known whether the Doctor rebuilt K-9 from scratch, or if he had Mark IV in storage. (DW: School Reunion)

The new K-9 referred to himself as being rebuilt and improved, suggesting Mark IV possesses the same memory as Mark III. (DW: School Reunion) Though Sarah Jane didn't find this out right away, the Doctor had also placed some gifts in a panel in K-9's side: sonic lipstick and a Scanner Watch that could detect alien life. (Sarah Jane Smith website) The sonic lipstick, in particular, would become an often-used tool in Smith's subsequent adventures (SJA: Invasion of the Bane, et al).

Sarah Jane would not be able to enjoy Mark IV's company for long. Approximately a year before Sarah Jane first met Maria Jackson (therefore not long after being presented to her), K-9 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.

K-9 was able to maintain occasion communication with Sarah Jane via a cabinet with a spacetime link when the orbit of the black hole intersected with that of Earth. (SJA: Invasion of the Bane) However, 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 to upload the Armaggedon Virus, (SJA: The Lost Boy) He was later called back to Bannerman Road in order to quickly upload the Doctor's TARDIS' base code to Mr Smith and assist the Doctor in returning the Earth to its correct location in space. (DW: Journey's End) It would appear that the spacetime link was left open between K-9's location by the black hole and Bannerman Road, as in this latter instance, Sarah Jane was able to summon K-9 verbally by broadcasting to her loft from the TARDIS.

After these events he went back monitoring the black hole only to find an unidentified member of the Slitheen family attempting to steal him as part of a revenge plot against Sarah Jane, who had foiled their plans in the past. K-9 banished the Slitheen, after reversing one of its attacks. (SJA: From Raxacoricofallapatorius With Love)

Later, Sarah Jane destroyed the black hole by transferring it into fuel for Eve's ship. K-9 then triumphantly returned to Bannerman Road to stay. He greeted Mr Smith (who held a grudge against him) and had a photo taken with Luke Smith, Clyde Langer and Rani Chandra. In 2059, Rani still had that photo. (SJA: The Mad Woman in the Attic) He then helped defeat Travist Polong and the Trickster. (SJA: The Wedding of Sarah Jane Smith) A drawing of him by Clyde was animated by alien energy and defeated the Abomination. (SJA: Mona Lisa's Revenge) He later assisted in tracking down a pair of Slitheen, locating them with his "olfactory sensors". (SJA: The Gift)

In 2056, Sarah left K-9 to a boy who would help later alongside with Drax to help search for the Sixth Doctor who was held captive by Omega. It is unknown whether it is Sarah Jane, or Luke, as Sarah Jane is presumably deceased by this time as she would be 105 if she was still alive, although, this is possible as Sarah Jane despite being 59 is still agile, and youthful whie fighting aliens on Earth (Search for the Doctor)

Features and Improvements
"Oi! Listen, in the year 5000, this was cutting edge!"

- The Tenth Doctor

This K-9 had a shiny metallic blue paint job and a number of improvements over Mark III including an omniflexible hyperlink facility. (DW: School Reunion) Later, though, K-9 reverted to his old, dark green color. (SJA: Invasion of the Bane) In episode 2 of The Lost Boy Sarah Jane unleashed K-9 Mark IV to battle Mr Smith. Before and after the battle K-9 displays a teleport facility and utilised a laser weapon similar to the previous K-9s'. (SJA: The Lost Boy) Later it was shown that he had a hover mode, but never show it (SJA: The Wedding of Sarah Jane Smith') 

Behind the Scenes

 * The designation K-9 Mark IV never actually appeared in dialogue. Documents packaged with the 1/4 scale Remote Control K-9 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 K-9's fleeing appearances in The Sarah Jane Adventures is due to other parties holding the rights to the character, for use in a spinoff TV series, K-9, although the character has not been completely forbidden from appearing in The Sarah Jane Adventures.
 * Search for the Doctor, a choose-your-own-adventure book published in 1986, features a future version of K-9 from the 2050s. At the time the book was published, this was implied to be K-9 Mark III, however given the events of School Reunion, it's now known that this is more likely Mark IV (or a later model as yet unrevealed), or due to the new K9 series it could be K9 Mark I