The Doctor makes a return visit to Peladon some years after his previous visit.
The Doctor brings Sarah to the Citadel of Peladon to introduce her to his friend the King, but accidentally arrives fifty years later to find his daughter Queen Thalira on the throne. He manages to avoid arrest by the timely arrival of an old friend, Alpha Centauri, now the Federation Ambassador to Peladon, who vouches for the Doctor’s identity despite the fact half a century has passed. It turns out that they have arrived during a very tense situation between the Galactic Federation, Pel miners, and Pel nobility. The Federation is at war with Galaxy 5, and desperately requires great amounts of the mineral trisilicate – a substance rich on Peladon – for the war effort. The miners feel exploited, their way of life having not improved in the half-century since Peladon joined the Federation. Leadership of the miners is divided between the centrist Gebek and the hotheaded Ettis.
Tensions rise further when the image of Aggedor begins to appear at random, attacking and killing miners. Chancellor Ortron takes this as a sign that their deity is displeased with alien intrusion in the affairs of Peladon. Ettis convinces the miners to strike and seizes a Federation armoury, while Gebek appeals for calm and negotiation. The Doctor suspects that someone is creating the appearances of Aggedor in order to further destabilise trisilicate production, but finds himself at odds with both the miners and Ortron, having only Alpha Centauri and Queen Thalira’s recollection of her father’s stories for support.
The Doctor and Blor, the Queen’s Champion, enter the tunnels to investigate Aggador’s last appearance. The Doctor finds a rich vein of trisilicate. There is a rock fall caused by the rebellious miners. Aggador appears and kills Blor, then turns on the Doctor.