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. 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-frought subterranean expedition to prevent a disaster that could destroy the very essence of Eskon... its cold heart.