Deep space, 5000AD. An Earth supply shuttle is infected by a strange sentient space-borne virus, which quickly spreads to the crew of the refuelling station on the moon Titan. Even worse, the Doctor himself falls victim. But why is Leela immune, and what is the virus's ultimate goal?
Elenco: William Hartnell , Patrick Troughton , Jon Pertwee , Tom Baker , Peter Davison , Colin Baker , Sylvester McCoy .