After Mark I sacrificed himself to destroy the Jixen warriors, Starkey retrieved his regeneration unit, which quickly activated. In a blast of white light, K9 was thus reborn as a much more advanced model, though this K9 Mark Z could not access the memories of his previous self, which had been "scrambled". He stayed at the house of Alistair Gryffen, who ran tests on K9, but did not tell the Department. K9 tried to kill Starkey, as he was sprayed with Jixen genetic matter and so registered as a Jixen. Starkey convinced him that he was not an enemy and K9 gave him his dog whistle to summon him when needed. (TV: Regeneration) K9 and the team tracked down the surviving Jixen to Dauntless Prison and killed it. They also faced a Meron and freed the alien prisoners. (TV: Liberation) K9 travelled with Starkey when he was on the run from the Department and rescued Gryffen (who was kidnapped by a Korven). Gryffen later welcomed him and Starkey into his home. (TV: The Korven)
K9 Mark III was a gift from the Doctor to Sarah Jane Smith (TV: A Girl's Best Friend). This K9 was broken for many years, until he was repaired by the Tenth Doctor. This K9 also had a "maximum defence mode", as well as offensive capabilities, such as a nose-mounted laser gun. He sacrificed himself to save Deffry Vale High School from Krillitanes. (TV: School Reunion)
K9 Mark I's mechanisms included rotating ear-probes, telescopic "eye" probe, an extendable nose concealing a powerful laser weapon, flashing lights on the top and the "eye" panel, waggling tail antennae, and ticker-tape tongue. His head could move up and down. His shell was painted in metallic gold/grey. There was a monitor screen on his left flank, though K9 rarely used this during his travels with the Doctor. Around K9's neck was a tartan collar, (being the Family Tartan of the Clan Buchanonn) from which hung a silver disk. (TV: The Invisible Enemy) He was later modified to fly through the vacuum of space. (TV: Curse of Anubis)
Either Inspector Thorne or Lomax found out about K9's regeneration unit and decided to acquire it. Thorne joined forces with Freddie Maxwell, owner of Crashclub involving old robots being made to fight. K9 went undercover as a gladiator and made friends with the robot clowns Chuckles and Boris. He was called to fight the Pain-Maker, who was laced with the explosive solarmite. When he was set to self-destruct, K9 detected this and fled the explosion. (TV: Robot Gladiators) To regain his memory and find out his connection to the STM, K9's archive was scrambled. Directives from the STM almost caused him to self-destruct and lose all his memory. However, with Gryffen and Starkey's help he overrode the directives. (TV: Mind Snap) He helped Gryffen find the STM's temporal stabiliser from the fallen angel crash site in Canada, where it was revealed to be Korven technology. (TV: Angel of the North) He stopped haywire CCPCs when The Last Precinct's plan backfired. (TV: The Last Precinct)

While caught up in a Time war between the past and the future on the planet Aoris, K9 was forced to spend a thousand years hidden on the planet, setting himself up as 'the great Machina' so that he could found a cult that would aid the Doctor and Romana in bringing the war to an end, hiding behind a fake computer with a version of the TARDIS until he could return to the Doctor and Romana once the war ended (AUDIO: The Paradox Planet, Legacy of Death).
After K9 Mark I regenerated into K9 Mark 2, his design was radically altered. His ear probes were replaced with larger silver aerials. A set of blue lights on his head flashed when he spoke. Most of his body was dark silver and a silver dog bone was located under his neck. He seemed to have the same capabilities as Mark I but was presumably more advanced. (TV: Regeneration)

After this he was tricked by Thorne to give him his regeneration unit for a lost memory chip. This turned him into a bomb to make him self-destruct on encountering Jixey. Starkey helped him disarm himself. (TV: Hound of the Korven) K9 Mark 2's brief life was almost cut tragically short when after the fall of the Korven invasion force, Thorne sent the Trojan to kill K9, who defeated it by burning out his power core and exhausting all his energy. As his regeneration unit was in the Trojan, K9 seemingly died at the mansion after his companions bade him farewell. However his regeneration unit homed in at the last minute and resurrected him, giving the frisky pup a new lease on life. The regeneration unit restored his old dog collar so he could celebrate the joyful occasion to the full as he flew off to more adventures, bristling with life. (TV: The Eclipse of the Korven)

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Jealous, Possessive by Paul Magrs, the "Scorpio" story in Big Finish's anthology Short Trips: Zodiac features K9 Mark I and Mark II relaying their exploits to each other, and their veiled put-downs to each other reveal that each considers the other to be the "inferior" version. This attitude is also occasionally hinted at in the way the two units refer to each other in the Gallifrey audio series.
Within the series' narrative, K9 is a robot dog acquired by Doctor Who's title character in the 1977 serial The Invisible Enemy. The first two incarnations of the character travelled alongside the Fourth Doctor (portrayed by Tom Baker) until 1981. In these stories, K9 proved useful for the powerful laser weapon concealed in his nose, his encyclopaedic knowledge and his vast computer intelligence. By 1981, each of the two models of K9 which travelled alongside the Doctor had been left with one of the Doctor's female companions. The character subsequently transitioned into spin-off territory. Producers hoped K9's popularity with children would launch the series K9 and Company, led by actress Elisabeth Sladen in the role of companion Sarah Jane Smith alongside a new K9. In 2006, K9 reappeared in the revived series of Doctor Who. Although the character's appearances in the 2007 spin-off The Sarah Jane Adventures were hampered by the concurrent development of the K9 spin-off, the character began to appear full-time in both K9 and The Sarah Jane Adventures, depicting adventures of the original K9 Mark I and of Mark IV, respectively.
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