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K9

 

Debuting in the 1977 serial The Invisible Enemy, the first K-9 was the creation of Professor Marius, a scientist working for the Bi-Al Foundation which was built on an asteroid near Titan in the year 5000. A mobile computer, K-9 was constructed in the shape of a dog as a substitute for the one Marius had left back on Earth. Highly intelligent, with an extensive database and equipped with sophisticated sensors as well as a laser weapon built into its nose, K-9 was instrumental in helping the Fourth Doctor and Leela defeat a sentient virus. At the end of the story, Marius suggested that K-9 join the Doctor on his travels.

 

All the K-9s referred to whoever owned them as "Master" or "Mistress" depending on their gender. The units were programmed to be both loyal and logical, with a penchant for taking orders literally, almost to a fault. The Fourth Doctor would often use a glib remark to disarm those who were surprised by K-9's appearance; in The Stones of Blood he said, "They're all the rage in Trenton, New Jersey." The Tenth Doctor defended its less-than-streamlined design ("...so disco!") to Rose Tyler, remarking that it was cutting edge in the year 5000.

 

Models

 

To date, four different versions of K-9 have appeared in the series:

 

K-9 Mark I continued to travel with and aid the Doctor and Leela until The Invasion of Time, when the Doctor left it on the planet Gallifrey to accompany Leela, who elected to remain behind on the Time Lord homeworld. However, once inside the TARDIS, the Doctor produced a box labelled "K-9 Mark II".

 

K-9 Mark II was more mobile than its predecessor, and exhibited the ability to sense and warn others of danger. It was with the Doctor and Romana when they were shunted into the parallel universe of E-Space, and was severely damaged by time winds during the events of Warriors' Gate. The damage was such that K-9 could only function in E-Space, and when Romana decided to stay and forge her own path, the Doctor gave K-9 to her. Due to a bout of laryngitis — which the Fourth Doctor was baffled as to how the robotic dog could have contracted — the voice of K-9 Mark II changes around the time of Romana's regeneration, only to revert some time later (in reality, this was due to voice actor Leeson temporarily leaving the series at the start of the 1979–80 season and being replaced by Brierley; Leeson returned for the 1980–81 season).

 

K-9 Mark III was a gift from the Doctor to Sarah Jane Smith, and appeared with her in the pilot episode of the aborted spin-off series K-9 and Company as well as briefly in the 20th anniversary television movie The Five Doctors. In the original outline to K-9 and Company, it would have been eventually revealed that K-9 Mark III was actually sent by and under the control of the Master, but this element never made it to the screen. K-9 Mark III's final appearance was in the second series of the new Doctor Who with David Tennant as the Tenth Doctor, alongside Elisabeth Sladen as Sarah Jane Smith in the episode "School Reunion". In the episode K-9 appears more run-down and worn, echoing Sarah's inability to repair it in the spin-off stories (see below). It died heroically, sacrificing itself by using the last of its energy reserves to set off an explosion.

 

K-9 Mark IV was a parting gift to Sarah Jane from the Doctor, appearing at the end of "School Reunion". It stated that the Doctor "rebuilt" it after the Mark III's sacrifice, implying that it had the same mind as the Mark III, but also confirmed it was a "brand new model", hence having the name Mark IV. Its systems were improved over those of its predecessor, including "omniflexible hyperlink facilities" and the ability to teleport. This model of K-9 also appeared briefly in "Invasion of the Bane", the pilot episode of The Sarah Jane Adventures, where it was shown sealing off an artificially created black hole. The real-world purpose of this is to remove K-9 from subsequent episodes of The Sarah Jane Adventures, in which he will not appear because of the concurrent development of K-9. K-9 reappeared once more in the series one finale to assist Sarah Jane against the series' villain. According to the official website for The Sarah Jane Adventures, the Doctor stored several presents for Sarah inside K-9, including a "sonic lipstick" and a watch to scan for alien life.