Yeo-Thomas was a staff officer who was sent on several missions by the Special Operations Executive (SOE) to France. Because the SOE and the Gaullist Free French organization, BCRA, seemed to be unproductively competing, Yeo-Thomas, as a Gaullist sympathizer working for the SOE, was sent to liaise between the two organizations in Paris and get them to accept central direction for their military activities. His first mission was in February 1943 when he was sent to Normandy to organize a Maquis there. In late 1943 he was sent on a mission with Pierre BROSSOLETTE to examine the security of the underground networks in Paris. He had to make contact with agents all over Paris and found himself being followed constantly—he was nearly arrested six times. On his third mission to rescue Brossolette, Yeo-Thomas was arrested the morning before his attempt to get him out of jail. He was betrayed by a subordinate and the Gestapo picked him up as he was waiting for his courier at the Passy metro. The Gestapo chained him up and tortured him and on one occasion immersed him repeatedly in an ice-cold bath. After a series of escape attempts he was transferred to Buchenwald concentration camp where he assumed the identity of a French Air Force officer, who had earlier died of typhus. On his first attempt to escape he was recaptured and put in an Oflag but he escaped again and reached Allied lines in April 1945.