Regeneration unit
The Regeneration circuit was a component of K-9 Mark I that enabled him to rebuild his body after death, but this could also scramble his memdrive. Seeing as other K-9s, or at least Mark III aren't fitted with a regeneration circuit, it can be assumed he was fitted with one on Gallifrey, presumably by Romana, it is unknown whether he remembers Leela or Romana, his two mistresses on Galifrey or if he remembers K9 Mark II, however it can be assumed that since K9 remembers who he is, he is likely to remember Romana and Leela, he apparently, doesn't remember the Doctor, but this may due to not seeing the Doctor for years, or that The Doctor was only a small memory and not programmed in his memory banks, unlike Leela and Romana, his two former mistresses. (K9TV: Regeneration)
