This is the novelisation of the story first broadcast in 1980 in Season 18, preceded by The Leisure Hive and followed by Full Circle. The story is novelised here by Terrance Dicks.
This is one of those stories that is not really very memorable; a bit two-dimensional and rather forgettable. On the planet of Tigella, the people live underground away from the rather dangerous plant life. Their source of energy, the mysterious Dodecahedron, is worshipped by one faction as a god, and utilised by another faction as a scientific artefact. When the power source begins to fail, their leader Zastor calls on the Doctor for help. But someone else is listening to the call for help and decides to use the Doctor's visit for their own purposes.
There is an attempt at humour in this story, which falls rather flat; and the menace which should be present to make the story work in this regard, is also missing. So, overall - a good story, but definitely not great; one for a diehard Doctor fan (like me). Promise holds for the next story, as the Doctor and Romana receive a message from Gallifrey and must respond.