The Normandy landings of June 1944 owed their success not only to the size and strength of the Allied forces, but also to the work of a little-known unit of the British army known as the Theatre Intelligence Service (TIS). From modest beginnings three years before, the TIS had evolved into a team of around three hundred investigators poring over maps, guidebooks, postcards and newspapers as well as Bletchley Park intercepts, aerial photographs and messages brought by carrier pigeons from occupied France. The TIS started issuing confidential weekly reports in 1942, and by 1944 it was ready with dozens of maps, plans and commentaries showing the condition of French beaches, the identities and attitudes of local politicians and the disposition of German forces to a depth of thirty miles. As D-day approached, it supplied essential intelligence not only to Supreme Headquarters but also, in thousands of copies, to troops as they went into action.

The TIS was an element in an enormous military machine, but it was also self-consciously unconventional: its members referred to themselves as Martians, their weekly dispatches were called Martian Reports and their invasion handbooks bore the skittish title Invade Mecum. The unit was, as M W Rowe puts it, a