A killer has been uncovered, but the mystery is far from solved. The Doctor, Leela, K9 and their friends are on the run, pursued from all sides. All the clues point to one place - but getting there alive may prove impossible. Something horrific is happening on Chaldera… and it has been happening for longer than anyone could possibly have realised. Now every life on the planet is at stake. Bar one. The dark secret at the heart of this world is about to be revealed.