Far out in space, an elite unit of soldiers is on a training mission. But deep in the heart of the hollow planetoid that forms the battleground, there is a chilling secret: ten alien corpses, frozen in time at the moment of violent, bloody death. The bodies are those of the empire's most wanted terrorists and their discovery could end a war of attrition devastating the galaxy. But is the same force that slaughtered them still lurking in the dark tunnels of the training ground? And what are its plans for the people of Earth? When the Doctor arrives on the planetoid with Ben and Polly, he soon scents a net tightening about them. And as the soldiers begin to disappear one by one, paranoia spreads; is the real enemy hiding in the dark, or is it among them?