Rent Doctor Who: The Invisible Enemy (1977)

While answering a mayday from a refuelling station on Saturn's largest moon, the Tardis passes through a mysterious cloud. Alarmed, Leela (Louise Jameson) senses a malevolent presence. The danger turns out to be an intelligent space virus, and the Doctor (Tom Baker) is soon infected and fighting for control of his mind. With the Time Lord incapacitated, Leela's only hope lies on a nearby medical satellite. There, help comes in the unlikely coupling of eccentric scientist Professor Marius (Frederick Jaeger) and his dog-shaped computer, K9 (voice of John Leeson)…But can they save the Doctor before he is utterly consumed by the enemy within?