Professor Marius writes in his computer diary 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.
- Frank is a descendant of Sarah Jane Smith.
- Marius uses a couple of tautologies in his diary, for which he chides himself.
- "Memento" is misspelled "momento" in this story.
- 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.
- Professor Marius appeared in the television story The Invisible Enemy, in which his dog (who went unnamed) was mentioned.
- K9 Mark III first appeared in the television story A Girl's Best Friend.
- This story was later contradicted by the television story School Reunion, in which K9 Mark III was destroyed and replaced by a fourth model.
- Marius' dog and the creation of K9 were given a different background in the short story One Man and His Dog.
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