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)
A short story, "Moving On", in Virgin Publishing's Decalog 3: Consequences anthology and the Big Finish Productions-produced Sarah Jane Smith audio play Mirror, Signal, Manoeuvre (both written by Peter Anghelides) indicate that K9 Mark III broke down and Sarah was unable to repair it as the replacement circuits would not be invented for several centuries.
K9 stayed with Leela on Gallifrey for many years. It was only after the disappearance of her Time Lord husband, Andred, that Leela and her K9 met Romana II and her K9. The two K9s did not get on well and competed. (AUDIO: Weapon of Choice) After Leela became Romana's bodyguard and after many important missions on her behalf, Leela's K9 was destroyed by an exploding bomb. Leela was heartbroken, but refused to have a duplicate unit built. (AUDIO: Imperiatrix)
K9 was a popular enough character to warrant an attempt to spin him off into his own series. In addition to K9 dolls, there were also talking K9 toys produced by Palitoy, the speech provided by a miniature record inside the body of the toy. John Leeson provided the voice for K9 in all its appearances, except for Season 17 (which included the unfinished Shada) when it was voiced by David Brierley. When Shada was remade by Big Finish Productions, Leeson provided its voice.
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.