Three philosophy professors travel to Prague for a conference. One of them, Anderson, is forced to rethink his ideas on ethics when a former student is arrested by the Czech authorities for writing about individualist approaches to morality.

The Doctor returns to Peladon fifty years after his last visit, to find Queen Thalira, daughter of the late King Peladon, on the throne. A tense labour dispute between Pel nobility and miners is worsened when apparitions of their deity Aggedor attack and kill several miners. The Galactic Federation desperately needs trisilicate for its war against Galaxy 5 and sends in brutal Ice Warrior troopers to ensure production. The Doctor discovers a devious plot at the heart of Aggedor's appearances.

Documentary about the tragedy which struck the Welsh mining village of Aberfan in 1966, when a mountain of coal slurry engulfed a school, claiming 144 lives, many of them children.