The Ice Warriors have taken control of Peladon, so the Doctor plans to defeat them.
The Ice Warrior takes the Doctor and Gebek prisoner. Another Ice Warrior, Azaxyr, appears in the Citadel and declares the planet under martial law. Azaxyr meets with all interested parties. He has no interest in the situation on Peladon, and only wants the trisilicate to be mined. He orders the miners back to work under armed guard and threat of death.
Meanwhile, a group of miners is planning a rescue of Gebek. They storm the throne room but are killed by the Ice Warriors. Ettis is the only survivor and runs away.
Azaxyr wants to execute the Doctor but Sarah, Alpha Centauri and Eckersley convince him that only the Doctor can convince Gebek to get the miners back to work. The Doctor suspects that their guard, Sskel, was the figure that Sarah saw at the refinery earlier, and Sarah confirms it. The Doctor and Alpha Centauri realise that Ice Warrior troops were on Peladon before Alpha Centauri sent for them.
The Doctor returns to the throne room to convince Gebek to start work, but the Pels are finally united against the Ice Warriors. The Doctor explains that he wants them to pretend to go back to work so he can work out a way to deal with the invaders. The miners return to work, pleasing Azaxyr, though he still doesn’t trust the Doctor. He has good reason to, as it turns out; the Doctor raises the temperature in the mines, which will weaken the heat-sensitive Ice Warriors and giving Gebek and his men a real chance of defeating them.
Ettis returns to the mines, but can’t be convinced that Gebek has a plan to defeat the Ice Warriors. Now insane, he wants to use the sonic lance to destroy the Citadel.
Azaxyr has not been fooled. He informs Thalira that he will wait on the situation before allowing her to contact Federation authorities.
The miners and Pel guards attack the Ice Warriors, who have been weakened by the raised temperatures in the mines. Gebek learns that Ettis intends to blow up the Citadel with the sonic lance; the Doctor will try to stop him. Sarah, looking after the miner that Ettis stabbed to get to the lance, is captured by an Ice Warrior and brought to Azaxyr. The Doctor reaches Ettis and fights with him to stop him using the lance. Azaxyr informs Sarah and Alpha Centauri that the sonic lance has a self-destruct that will engage if Ettis tries to use it. Ettis defeats the Doctor and starts the sonic lance. It explodes.