The Doctor and Nasreen Chaudry go deep underground and find an ancient society that has been disturbed by the drilling. They are gassed and taken prisoner just as Amy manages to free herself. The military commander of the underground reptilians, Restac, is convinced that the "apes' - their term for humans - are an invading force. Calmer heads prevail and the Doctor manages to negotiate a truce pending the transfer of prisoners which he, and some of the more pacifist members of the reptilians, hopes will lead to a lasting peace. The only problem is that Alaya, the reptile prisoner has been killed and Restac is ready to go to war.