While answering a mayday from a refuelling station on Saturn’s largest moon, the TARDIS passes through a mysterious cloud. Alarmed, Leela senses a malevolent presence. The danger turns out to be an intelligent space virus, and the Doctor 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 and his dog-shaped computer, K9… But can they save the Doctor before he is utterly consumed by the enemy within?