Over in the tiny, remote hamlet of Cwmtaff, human science is again being bold, pushing boundaries and asking for trouble. The Discovery Drilling Project - or as the technically-minded Doctor calls it, "a Big Mining Thing" - has burrowed 21 kilometres into the Earth, deeper than ever before. However, as Discovery boss Nasreen Choudhry and her project manager / unrequited love Tony Mack celebrate, an ancient race launch pre-emptive strikes, from way further beneath their feet than 21 kilometres. People alive and dead start to vanish. The Doctor, Amy and Rory face the Ultimate uprising...