"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.

The medical center mounts a defense against alien forces trying to take the comatose Doctor while clones of Leela and the Doctor go microscopic so they can hunt down the intelligent infection that's in his head.