While caught up in a Time war between the past and the future on the planet Aoris, K9 was forced to spend a thousand years hidden on the planet, setting himself up as 'the great Machina' so that he could found a cult that would aid the Doctor and Romana in bringing the war to an end, hiding behind a fake computer with a version of the TARDIS until he could return to the Doctor and Romana once the war ended (AUDIO: The Paradox Planet, Legacy of Death).
Before this, the Doctor obtained or constructed at least one backup model of K9. Immediately after leaving Mark I behind with Leela, he unpacked K9 Mark II. This version of K9 accompanied the Doctor and his new companion on their quest to locate the segments of the Key to Time, (TV: The Ribos Operation) but the swampy terrain of Delta III made it impossible for him to join them on the planet. At the conclusion of the quest, the Shadow turned K9 to his evil will, giving K9 a sinister personality. (TV: The Armageddon Factor)
Within the series' narrative, K9 is a robot dog acquired by Doctor Who's title character in the 1977 serial The Invisible Enemy. The first two incarnations of the character travelled alongside the Fourth Doctor (portrayed by Tom Baker) until 1981. In these stories, K9 proved useful for the powerful laser weapon concealed in his nose, his encyclopaedic knowledge and his vast computer intelligence. By 1981, each of the two models of K9 which travelled alongside the Doctor had been left with one of the Doctor's female companions. The character subsequently transitioned into spin-off territory. Producers hoped K9's popularity with children would launch the series K9 and Company, led by actress Elisabeth Sladen in the role of companion Sarah Jane Smith alongside a new K9. In 2006, K9 reappeared in the revived series of Doctor Who. Although the character's appearances in the 2007 spin-off The Sarah Jane Adventures were hampered by the concurrent development of the K9 spin-off, the character began to appear full-time in both K9 and The Sarah Jane Adventures, depicting adventures of the original K9 Mark I and of Mark IV, respectively.