The Doctor tries to get everyone to Rio, but nothing quite goes as planned. They arrive in a small Welsh village where the Doctor immediately senses that the ground isn't quite right. There is a drilling rig nearby that has reached a depth of 21 kilometers when odd things begin to happen. A local village man has disappeared and soon after Amy is swallowed into the ground. The Doctor realizes that the drilling has disturbed an ancient reptilian race that has lived deep underground for thousands upon thousands of years and who are determined to protect themselves against the 'apes' who live on the surface.
- In 2020, the most ambitious drilling project in history is under way. Dr Nasreen Chaudry and her team have reached 21 kilometers into the Earth's crust - but something is stirring far below. Amy Pond discovers there's nowhere to run when you can't even trust the ground at your feet.