The Fox, a brilliant German agent, patiently bides his time as Allied forces prepare for the invasion of Nazi-occupied France.  Southern England is one huge armed camp and no-one is aware of the danger in their midst.  A sudden, deadly E-boat attack stuns the military.  Top secret invasion plans are stolen.  The Fox has struck.  As he tries to get the plans to Germany in time, two officers are ordered to stop him at all costs.