"The Krill are pure rage. Pure aggression. There is nothing for me to reason with, nothing that I can appeal to. By now there must be millions of them out there, and I don't know how to stop them."

The water world of Coralee is the showpiece planet of the colonies — two suns, unspoilt beaches and a thriving tourist culture. The Doctor and Ace arrive intending to have a proper holiday, but when the Doctor joins an archaeological expedition conducting a dig at a sunken temple, he soon realises that, beneath the surface, something is very wrong. An ancient and terrible weapon has been revived and every living creature on the planet is at risk.

Who are the aliens that wait in the asteroid field? What is the mysterious creature that stalks the shadows of the colonies? Who has unleashed the devastating power of the Krill?

Out in the deep ocean there is a storm brewing, and the Doctor is not sure if anyone will survive.