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Professor Marius was a scientist who worked at the Bi-Al Foundation in the year 5000 as a specialist in extraterrestrial pathological endomorphisms. He was the creator and first master of K9 Mark I.
While off Earth he also wanted a dog so he constructed K9 to accompany and assist him in his work. He helped the Fourth Doctor defeat the Swarm, although he himself was briefly infected by it and so freed the Nucleus and made it grow in size. Afterwards, Marius gave K9 to the Doctor to take with him on his journeys. (TV: The Invisible Enemy)