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.

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.