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)
K9 dogs are expected to perform well in various situations, such as finding a suspect through smell, protecting its handler, finding hidden objects in a 150’x150′ area, knowing how to respond when under fire, and detecting explosives and narcotics. Before a handler can start working with a dog as an official Police dog, the dog must pass the normative police test, or can be bought, fully prepared, from Master Dog Training.
During training at Master Dog Training, a K9 officer will better understand his or her dog, and the police dog will flawlessly perform its duties. During work and practice, the handler will be able to improve his or her dog’s performance, rather than ruin it by doing something incorrectly. For a K9 Police officer, we will solve any behavior problems that the dog might be displaying, and will teach the handler how to correct these problems during work. During the K9 police test, we will be teaching how to properly pass it through all the different disciplines.
In 1990, an unspecified K9 unit appeared with Sylvester McCoy as the Seventh Doctor and Sophie Aldred as Ace in an episode of the children's education programme Search Out Science entitled Search Out Space, which was included as an extra on the release of the Doctor Who story Survival by the BBC. Another unspecified K9 unit also appeared in the 1993 charity special Dimensions in Time. In the 1998 computer game Fallout 2, the Navarro base has a damaged robot dog known as K9, which uses similar speech mannerisms to the Infobox Doctor Who character. If repaired, the cyberdog is willing to join the character's party as a companion. In the 1999 television series Queer as Folk (written by future Doctor Who executive producer Russell T Davies), a K9 model is given to the character Vince as a birthday present. The prop used was an original, operated — as occasionally in Doctor Who — by visual effects assistant Mat Irvine. In the second series of I'm Alan Partridge (2002), the character of Alan Partridge recalls how his purchase of the rights to K9 contributed to his mental breakdown and driving to Dundee in his bare feet while gorging on Toblerone. In the South Park episode "Go God Go XII" (2006), Eric Cartman, being trapped in the year 2546, has acquired a robot dog called "K-10", a parody of K9. Due to timeline alterations, he is replaced by robot cat "Kit-9" and later robot bird, "Cocka-3". K9 appeared on a special Doctor Who-themed edition of The Weakest Link in 2007, but was voted out unanimously at the end of the first round, despite answering his question correctly. This was due to the fact every single player of the team answered correctly and banked the £5,000 target, and that the producers told the contestants to vote him off, just in case he broke down. Anne Robinson (whom K9 addressed as "Mistress") said "I'm so sorry" before declaring him the weakest link.[citation needed]
Dallas K-9 Academy is the best and the most effective dog training available. Your dog is trained in your home (or place of business). We also offer board and train programs. Our effective techniques developed through more than 18 years of experience help you teach your dog to understand how he should behave in day-to-day situations. At the completion of training, we deliver your dog to your home and work diligently with all family members to assure you he will continue his newly learned behaviors.

K9 Mark I, II, III, and IV addressed whoever was directing them as "Master" or "Mistress" depending upon gender, and used the formal "affirmative" and "negative" rather than "yes" and "no". They were programmed to be both loyal and logical, with a penchant for taking orders literally, almost to a fault. Their striped collars mirrored the Fourth Doctor's trademark scarf.
He usually did not engage in computer-like literal thinking, though on one occasion he followed Romana's instructions to "forget it" to the letter and erased all knowledge of tennis from his memory banks. (TV: The Stones of Blood) In another he took Romana's exclamation "Blast it!" as a command, and fired his blaster. (TV: Shada, WC: Shada, AUDIO: Shada, PROSE: Shada). Despite this, he occasionally showed a sense of humour, jokingly answering the question "What do you eat?" with "Ball bearings" (AUDIO: The Beast of Kravenos).
At Solid K9 Training, we understand the importance of developing a unique real world dog training program that brings out each dog’s full potential so they can exist in harmony with their owners each and every day. We are not just focused on teaching your dog obedience commands, but more importantly to make good choices on their own, access a calm and relaxed state of mind, and live harmoniously with you, as their leader and guide. When all of these things are in place, you will see a significant improvement in your dog, and your relationship will transform into the type of bond you may have never imagined possible. Solid K9 is one of a handful of trainers that, not only WILL work with, but successfully trains and rehabilitates aggressive dogs that have a history of biting other dogs, and/or people.

Introduced into the plot in The Invasion of Time, K9 Mark II first actually appear on-screen in The Ribos Operation (1978). More mobile than his predecessor, Mark II exhibited the ability to sense and warn others of danger. He travels as a companion alongside the Doctor and Romana. Around the time of Romana's regeneration (the transition from actress Mary Tamm to Lalla Ward), the K9 character was explained as suffering from "laryngitis" to accommodate Leeson's departure from the series at the start of the 1979–80 season; for this time, he was portrayed by David Brierley, until Leeson's return for the 1980–81 season. When the Doctor and Romana travel to the parallel universe of E-Space, K9 is severely damaged, in Warriors' Gate (1981). The damage was such that K9 could only function in E-Space; when Romana decided to stay and forge her own path, the Doctor gave K9 to her. The character makes subsequent appearances in remakes of the unfinished serial Shada, alongside the Eighth Doctor (Paul McGann); the 2003 Shada audio play and webcast depict K9 Mark II as having returned to Gallifrey with Romana, now Lady President of the Time Lords, where the Doctor visits them. It is not clear if either of these audio appearances are considered canonical in relation to the continuity of the TV series.

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