The Doctor, Fitz and Compassion arrive on the planet Eskon — a strange world of ice and fire. Far beneath the planet's burning surface are vast lakes frozen solid by the glacial subterranean temperature.

But the civilised community that relies on the ice reservoirs for its survival has more to worry about than a shortage of water. The hideous slimers — degenerate mutations in the population — are growing more hostile by the moment, and their fanatical leader will stop at nothing to exact revenge against those in authority. But what connects the slimers to the unknown horror that lurks deep beneath the ice? And what is the terrible truth that the city leaders will do anything to conceal?

To unearth the ugliest secrets of Eskon, the TARDIS crew becomes involved in a desperate conflict. While Fitz is embroiled in the deadly plans of the slimers, the Doctor and Compassion must lead a danger-fraught subterranean expedition to prevent a disaster that could destroy the very essence of Eskon... its cold heart.