Despite those drawbacks, K9 comes with a handful or so of predesigned filters and an option to customize. With more than 50 categories for organizing Web sites, and the keyword-free proprietary K9 rating system, the Web monitoring and blocking aspects of the software functioned well. K9 also has categories for blocking sites that have been detected as potential malware threats. Equally impressive, and a little bit scary, was the log that detailed not just blocked Web sites but also every Web site visited.
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)
K9 Mark IV was a recreation and enhancement of the Mark III created by the Tenth Doctor as a second present for Sarah Jane. (TV: School Reunion) This model continued in service to his mistress, Luke Smith, Clyde Langer, and Rani Chandra and was known by Maria Jackson and Kelsey Hooper. He was left fixing a black hole in Sweden for many years, but after returning to Earth, was used sparingly, until he went to university with Luke Smith. (TV: The Nightmare Man)

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)
The Eighth Doctor Adventures novel Interference: Book Two by Lawrence Miles indicates that the Doctor built a Mark IV model sometime prior to the events of that book, but what happened to this unit is not stated. In the novel The Gallifrey Chronicles by Lance Parkin, K9 Mark II makes another appearance, having been trapped within the TARDIS since the events of The Ancestor Cell. At the end of The Gallifrey Chronicles the Doctor sends K9 on a secret mission to Espero, presumably to seek out his former companion, the living TARDIS known as Compassion.
K9 Mark I was created about the year 5000 by Professor Frederick Marius, who gave the dog to the Fourth Doctor and Leela. He was left behind with Leela on Gallifrey. (TV: The Invasion of Time) Some time later, he was blown up by a bomb on Gallifrey, (AUDIO: Imperiatrix) but survived. This model came to an end when Alistair Gryffen's Space-Time Manipulator randomly pulled him to London in the year 2050. There, he almost immediately self-destructed to protect Gryffen and a group of teenagers from Jixen warriors. (TV: Regeneration)
The Fourth Doctor found that K9 had contracted laryngitis and had lost his voice as a result of this. K9 was left in the TARDIS while the Doctor and Romana II battled the Daleks and Davros. (TV: Destiny of the Daleks) During a later adventure, he remained patiently in the TARDIS, which would imply that his voice had not yet returned. (TV: City of Death) After his voice returned, it sounded quite different and remained so for a while (TV: The Creature from the Pit) before the familiar voice returned. (TV: The Leisure Hive)
Consequences that may occur from this is that the dog ends up not being able to work well with the police in a certain situation, which can have extremely serious effects. In this case, the police must state the problem to the personal dog trainer, so that the K9 trainer can teach the dog and the K9 handler how to fix any mistakes that could have been made by the handler.
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Although the first incarnation of K9 does not appear again in televised Doctor Who media, he is the star of the 2009 K9 television series, in which he undergoes a sort of "regeneration" process from which a new, more sophisticated and futuristic K9 emerges;[1] in the first series of K9, the character is transported to London c. 2050 by Professor Gryffen (Robert Moloney). Though on regeneration the character loses his memory of his adventures with the Doctor, he assists Gryffen and several teenage companions against a dystopian regime of "The Department". The upgraded K9 has new specifications, sporting a sophisticated head-up display, the ability to fly, and more powerful laser weapons.
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