Unlike 25 years earlier where she appeared unconcerned with K9 Mark III being seen, Sarah Jane restricted K9 Mark IV to her Ealing house except in emergencies or when she required his services, lest she have to explain him. Her son Luke covered by explaining to a puzzled Peter Dalton that K9 was a Japanese robot toy with pre-programmed phrases. (TV: The Wedding of Sarah Jane Smith) Nevertheless, she sent K9 to serve Luke when he went away to the University of Oxford. (TV: The Nightmare Man)

K9, retroactively "K9 Mark I" (Leeson), initially appeared in The Invisible Enemy (1977) as the creation of Professor Marius (Frederick Jaeger) in the year 5000. K9 subsequently travelled with the Fourth Doctor (Tom Baker) and Leela (Louise Jameson) as a companion of the Doctor in his adventures in time and space until The Invasion of Time (1978). In this serial, K9 decides to remain on the Doctor's home planet of Gallifrey with Leela. Immediately afterwards, Doctor Who would introduce a second incarnation of K9, played by the same prop; the last scene of The Invasion of Time shows the Doctor unpacking a box labeled "K9 Mk II".
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