The Doctor and Evelyn arrive in Cornwall and encounter a very special old friend, apparently helping out with a local archaelogical dig. But Brigadier Lethbridge-Stewart has another agenda which has been laid down by his old employers, UNIT.

Evelyn meanwhile strikes up a friendship with the local aristocracy in the form of Sir Archibald Flint and the Doctor decides to go digging about in an old fogou.

However, something very ancient and very evil is gathering up its power in and around Lanyon Moor and before long the Doctor discovers that not only does everyone in the village harbour secrets, some of those secrets could lead to the destruction of the planet..