In search of ‘exotic particles’, the Doctor and Flip arrive 700 feet underground, in a mine in Northern Mexico – only to run into a scientific expedition. Among their number, an exobiologist. They’re all on the hunt for alien life! Deep underground, the team finally uncovers a cave of vast crystals – like ice, despite the heat. And inside the crystal: something frozen. Something trapped in time. If only it were something simple, like a monster. But it’s far, far worse than that.