In the year 5000 AD a three-man crew arrive on the planet Titan but unknowingly become hosts for an alien virus that turns them hostile. Picking up a distress signal in the Tardis, the Doctor and Leela head to the planet to investigate.
"Contact has been made!" A relief crew en route to Titan in 5000 AD unwarily pick up hitchhikers that change them. Shortly thereafter the Doctor, passing through the same point in space, is hitchhiked as well, but not Leela - a reject who must die.—statmanjeff