"My name is Bliss," said the newcomer, "and I bring great news for you all!"

The new owners of a Second World War aerodrome promise a golden dawn of prosperity for the East Anglian village of Culverton. The population rejoices — with one or two exceptions. Former Spitfire pilot Alec Whistler knows the aerodrome of old, having found a strange, jade-coloured crystal there years before...

When black-shirted troops appear on the streets, Whistler takes his suspicions to his old friend Brigadier Lethbridge-Stewart. The Doctor and Jo are sent to investigate and soon discover that all is not well in the seemingly idyllic village.

What are the black coffin-like objects being unloaded at the aerodrome? What horror lies behind Legion International's impeccable facade? And what is the monstrous creature growing and mutating in the marsh?

As Culverton gears up for its summer fete, the Doctor finds himself involved in a race against time to prevent a massive colonisation of Earth. For the last of the Gaderene are on their way...