Dan Snow investigates an extraordinary network of secret cells and underground bunkers built during Britain's "darkest hour".

In the summer of 1940, Britain and its empire stood alone as the Nazi war machine stormed through Europe. But in its darkest hour, Prime Minister Winston Churchill created something extraordinary: a secret network of men trained not to survive but to wreak havoc behind the lines of an invading army.

Dan joins forces with Andy Chatterton from the Coleshill Auxiliary Research Team, who are unearthing this incredible part of World War Two.