As D-Day nears, Hitler approves a desperate scheme to derail the invasion. The Allied Naval Command's headquarters at Dover Castle is vulnerable. Three neophytes at MI5 sniff the plot but London ignores their warnings, unable to admit a flaw in their vaunted Double-cross spy cadre. With time running out, the trio must find a way to stop the Nazis and prevent the Normandy Landings from collapsing in disarray.