The TARDIS materialises inside an Aztec tomb. Outside it, the Doctor and his companions soon discover that Mexico in the 15th century is a bloodthirsty and dangerous place... And with Barbara mistaken for a reincarnation of an ancient high priest called Yetaxa, the history teacher thinks that she can put an end to the barbaric human sacrifices once and for all. But can she rewrite history without disastrous consequences?

威尔和他的父亲在一座孤立的山上过着简单的田园生活。当威尔的父亲被一头龙杀死后，威尔必须动身下山，并根据一个始于多年前的神秘的家族联系去护卫斯特林，一个曾经富有的领主和骑士。他的到来并不受欢迎，威尔被安排到艰苦而又山路漫漫的农场去干活。他很快成为斯特林任性女儿凯特的朋友。他们两个开始共同努力来揭开连接他们两家的秘密，并帮助威尔准备他与龙的决战，在这决战中，他将向杀死他父亲的龙复仇，并且证明自己是一个男人。