Someday K9 saved the people at the Aurora from being killed by a Federation war cruiser. (PROSE: Peaceniks) When K9 wanted to help a space ship navigating back to New Earth he met Interstitials. These creatures were living in a state relating to the space or time between things. K9 helped them pass into another existence where it was better for them. (PROSE: Interstitials)
Almost as soon as he arrived, K9 Mark IV left Sarah Jane to stabilise a dangerous black hole in space which had been created in Switzerland. He could only speak with Sarah Jane occasionally via a cabinet with a space-time link when the orbit of the black hole intersected with Earth's and did not know when he would complete the job. (TV: Invasion of the Bane) However, he returned to Earth very briefly during emergencies. He did so on two later occasions in order to render vital assistance. The first time, he fought Mr Smith in pitched battle to give Sarah Jane time to upload the Armageddon Virus. (TV: The Lost Boy) The second time, he quickly uploaded TARDIS base codes to Mr Smith, assisting the Tenth Doctor in returning Earth to its correct location in space. (TV: Journey's End)
On 3 April 2010, the TV series K9 began broadcasting on the Australian Television Network Ten as part of a Saturday morning line-up of children's shows. John Leeson is the voice of K9. The first episode called "Regeneration"[13] featured the earlier Mark I version of K9. The "older" K9 materializes in London in the future and in the midst of an incident where an alien species is threatening human lives. K9 sacrifices himself in order to save the humans, but one component survives that enables his self-regeneration into the new unit. The main human cast members include Robert Moloney as Professor Gryffen, Keegan Joyce as Starkey (K9's new "master"), and Daniel Webber as Darius. The first 14 episodes of K9 season 1 were broadcast mid-morning on Channel 5 in December 2010 with the remaining episodes scheduled for January 2011. Trailers for the series appeared on Channel 5 from early December voiced by John Leeson. Originally, it had been planned to split the series in two halves between the Christmas 2010 and Easter 2011 school holiday schedules.

To run the program, you must register on the Web site to get the free activation code. The control panel is accessible only through the Internet. Uninstalling requires removing the app from the Task Manager, then using its password to delete it from the Windows Control Panel, and finally, rebooting. Because the password gets sent to the e-mail account of the person who registered the program, it's possible that an enterprising user could disable K9 on a shared computer.
Later he defeated Ahab, who tried to convince him he had murdered Zanthus Pia. He became part of the K9 Unit, being made the responsibility of Gryffen and under the control of Department inspector June Turner. (TV: The Bounty Hunter) He foiled Inspector Drake's plan to control the population using Cerulium (TV: Sirens of Ceres) and experienced fear. (TV: Fear Itself) He aided in the defeat of echoes of Gryffen's family, (TV: The Fall of the House of Gryffen) Orthrus, a duplicate of him created by Drake to frame him (TV: Jaws of Orthrus) and the Bodach. (TV: Dream-Eaters) He met Nehetka and Geb of the Anubians, a race his previous incarnation liberated. He was so obsessed with regaining his lost memories that he didn't notice Gryffen, Starkey and Jorjie's takeover by the Anubians. He soon found out when the Anubians attacked the Department. With Darius' help, he defeated them. (TV: Curse of Anubis)
K9 was not originally intended to be a companion, but producer Graham Williams liked the concept so much that the decision was made to retain him as a regular character, in order to appeal to the younger members of the audience. The original name for the character was "FIDO" — apparently from "Phenomenal [sic] Indication Data Observation" unit — but it was eventually named K9.

K9, retroactively "K9 Mark I" (Leeson), initially appeared in The Invisible Enemy (1977) as the creation of Professor Marius (Frederick Jaeger) in the year 5000. K9 subsequently travelled with the Fourth Doctor (Tom Baker) and Leela (Louise Jameson) as a companion of the Doctor in his adventures in time and space until The Invasion of Time (1978). In this serial, K9 decides to remain on the Doctor's home planet of Gallifrey with Leela. Immediately afterwards, Doctor Who would introduce a second incarnation of K9, played by the same prop; the last scene of The Invasion of Time shows the Doctor unpacking a box labeled "K9 Mk II".

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