'The Doctor, Nyssa and Tegan answer a distress call from Earth colony F-four, which has been cut off from the rest of the galaxy for ten years. No children have been born in that time. Within a day, Tegan has come down with a terrible chill and fever, and when the Doctor runs tests he discovers that every colonist is carrying a lethal virus -- and that the last crop shipment sent to the colony was genetically engineered to render the colonists sterile. It seems that the planet was seeded with the virus by an alien species which feared that humanity would expand into its territory; the colonists are carriers, and if any of them get off-world, billions will die. The colonists refuse to accept this and try to force the Doctor to take their representatives off-world, but the Doctor takes Tegan aboard and dematerialise, tricking the colonists into believing that he has self-destructed the TARDIS rather than risk taking them away. The colonists realise that he would never have taken Tegan aboard if he intended to do so, but when they run their own tests they confirm the Doctor's findings, and release Nyssa, realising that they have no hope. The Doctor is able to cure Tegan, but the virus has embedded itself in the colonists' DNA; they will remain infected and unable to bear children, and the colony will slowly die out in isolation.'
[The Doctor is the Fifth Doctor.]
Source: drwhoguide.com (http://www.drwhoguide.com/whotrip8.htm). Sighted: 20/5/11