ProfessorMarius writes in his computerdiary that he has been posted to a station close to the space lanes, and anticipates being bored there. He acquires an old computer from his niece's fiancé. Strangely, although it is from the 20th century, its technology is far in advance of that time. However, due to shuttle weight restrictions he is forced to not only have it scrapped, but also to leave behind his dog Toby. He keeps the computer's motherboard and Toby's dog tag as mementos. At the station, he befriends an electronic specialist who helps him construct a new computer based on the motherboard and sketches Marius drew. The professor plans to name it after the old computer: K9.
This story reveals that K9's existence is the result of an ontological paradox, as Marius gets the inspiration to construct K9 Mark I from his examination of K9 Mark III, who was himself constructed or obtained by the Doctor after meeting K9 Mark I.