Marcel Eusèbe Defence was born in 1920 and grew up in Glasgow. The son of a French father and a Belgian mother, he attended a local boys' school and finished his education at a French college in Epernay. Moving to England, he took up an apprenticeship with the Austin Motor Company but decided to join the Territorials in April 1939; after the outbreak of war he went on to serve with the Royal Army Service Corps in France, during which he suffered a serious injury in a motor cycle accident, losing a knee cap. In May 1942 he joined the Intelligence Corps, and in September was recruited by F Section.

His training reports were mixed, describing a tough, competitive character who followed instructions to the letter but lacked any talent for spontaneous action. Although there were question marks over his French and his knee problem, his quiet and unobtrusive nature recommended him for the role of wireless operator, and he was parachuted to the SCIENTIST circuit in May 1943.

The organiser of SCIENTIST, Claude de Baissac, had been working with a Resistance leader named André Grandclément, whose OCM movement was receiving large quantities of SOE's arms and supplies. De Baissac had also supported a sub-network in Paris managed by Grandclément's OCM comrade Marc O'Neill (a Frenchman, despite his Irish surname), and in early August Defence was sent to work for him. This was an especially dangerous assignment, since F Section's PROSPER circuit had collapsed in Paris two months earlier, and some of O'Neill's contacts had been connected with both organisations.

In September Grandclément travelled to meet O'Neill but was arrested just a few hours before their rendezvous. The Gestapo managed to persuade him to work against the Resistance, and Defence and O'Neill were forced to relocate, Defence returning to Bordeaux. Later that week Grandclément met Defence and Roger Landes to declare his intentions, and although Landes threatened to kill him, he walked away unscathed. Grandclément's subsequent collaboration with the Germans rapidly led to the demise of SCIENTIST, and its agents had little choice but to return to England. After an unsuccesful attempt to enter Spain, Defence eventually crossed the Channel and arrived back in London in January 1944.

In March he volunteered for a second mission, to establish the SATIRIST circuit north of Paris. Parachuting with SATIRIST's organiser Octave Simon, they had expected to be met by a reception committee arranged by Francis Garel's BUTLER circuit, but instead they were dropped straight into the waiting arms of the Gestapo, victims of a 'radio game': Garel and his wireless operator had been captured six months earlier, and the Germans had been operating BUTLER's captured wireless sets to dupe F Section into sending more agents and supplies.

After being held at the Gestapo prison at Place des Etats-Unis in Paris, Defence was deported to Gross Rosen concentration camp in Silesia, where he and eighteen other SOE agents were executed by firing squad in July 1944. He was mentioned in despatches in 1945. His name appears on the F Section memorial at Valençay in France, and the SOE memorial at Gross Rosen.