The aerodrome in Culverton has new owners, and they promise an era of prosperity for the idyllic village. But former Spitfire pilot Alex Whistler is suspicious — when black-shirted troops appear on the streets, he contacts his old friend Brigadier Lethbridge-Stewart at U.N.I.T. The Third Doctor is sent to investigate — and soon uncovers a sinister plot to colonise the Earth. The Gaderene are on their way.