The Doctor and Nyssa start doing biological tests. Tegan, feeling a bit left out, talks with the colonists about their harsh life. She shares a meal with them. That night, she becomes seriously ill.
The Doctor rules out food poisoning. Nyssa and one of the colonists, Toopyar, start testing the crops, thinking they might have absorbed some toxin from the soil.
They discover that someone has placed a contraceptive gene in the original seed supplies. It would stop a pregnancy that might occur, and even if the mother fasted, once she started to eat again, she would miscarry. Toopyar informs Nyssa that the native food sources are very meager.
Tegan wakes in the medical hut to find the Doctor barricading the door. He calls out to the colonists outside that Tegan is ill, not from the contraceptive in the food, but from a genetically engineered virus that also appears in Toopyar's blood. Tegan would have died if it weren't for the Doctor's advanced medicine. The colonists try to break in, wanting the Doctor to take them away from the colony, but he can't until he finds out what virus they are carrying.
Tegan wakes later to find herself alone. Still in the stupor from her illness, she figures it out. She makes her way to a meeting place where the colonists are badgering the Doctor to take them off the planet. Tegan pushes forward and addresses the crowd. She tells them that someone put the disease here before they arrived, hoping the colonists, as carriers, would take the virus back to Earth. The genetically altered seeds were sent by those on Earth who realised that the colonists were carriers of a deadly disease.
The colonists pull guns on the travellers and order them to take them off the planet. The Doctor agrees to take two of them to the TARDIS, on the condition that he can also take Tegan.
As the four head for the TARDIS, they are followed by many of the colonists. The Doctor tells them he has to get Tegan inside first, but once the two are in the TARDIS, he dematerialises, out of sight but close enough to monitor events on the scanner. He and Tegan watch as Nyssa tells the colonists that the Doctor destroyed the TARDIS rather than taking them with him.
The Doctor explains to Tegan that he and Nyssa arranged this plan earlier, and that Nyssa will join them after dark. However, Nyssa appears earlier than expected; the colonists knew the Doctor wouldn't have killed Tegan. They performed tests themselves and confirmed the Doctor and Nyssa's findings, and then let Nyssa go, asking her to tell their story to anyone who will listen.
Tegan, on the brink of unconsciousness, promises that she will tell their story.
- All Tegan knows about science comes from The Curiosity Show.
- She does, however, know how to evacuate a jumbo jet in ninety seconds.
- The Doctor has advanced medicine on the TARDIS that cures Tegan of the deadly disease.
- The colonists speculate as to who put the disease on F-four, guessing that it could have been the Daleks or the Draconians.
- The stories in Short Trips: Steel Skies are divided into four sections dealing with different types of confinement. This story deals with frontiers, that is, "the corridors, living quarters and ventilation shafts of four futuristic environments".
to be added