Donald Darling, code named Sunday, was an agent for the clandestine British organizations MI6 and MI9 during World War II. The purpose of MI9 was to help prisoners of war to escape and downed airmen and stranded soldiers to evade capture in German-occupied Europe and return to Great Britain. Darling worked in Lisbon and Gibraltar. He financed and advised the escape and evasion lines which rescued soldiers and airmen and guided them to safety in neutral Portugal and Spain and British-owned Gibraltar. The escape lines rescued 7,000 soldiers and airmen in western Europe. Darling met and interviewed many of them on their arrival in Portugal and Gibraltar. As part of his work, Darling contributed intelligence to MI6 about conditions and events inside occupied Europe through knowing many of the key people involved in resistance and escape lines.

MI9, the British Directorate of Military Intelligence Section 9, was a secret department of the War Office between 1939 and 1945. During World War II it had two principal tasks: assisting in the escape of Allied prisoners of war (POWs) held by the Axis countries, especially Nazi Germany; and helping Allied military personnel, especially downed airmen, evade capture after they were shot down or trapped behind enemy lines in Axis-occupied countries. During World War II, about 35,000 Allied military personnel, many helped by MI9, escaped POW camps or evaded capture and made their way to Allied or neutral countries after being trapped behind enemy lines.