On 28 February 1940, Grover-Williams enlisted in the British
Royal Army Service Corps and worked as a driver in France. In June 1940, he was
evacuated from Dunkirk to England after the victory by
Nazi Germany in the
Battle of France. Due to his fluency in both French and English, he was recruited into the Special Operations Executive (SOE) in autumn 1940. SOE trained him to be an agent and promoted him from private to second lieutenant. On 29 May 1942, Grover-Williams and another SOE agent,
Christopher Burney, parachuted "blind" (with no reception committee on the ground) into France near
Le Mans. Grover-Williams proceeded to Paris where he reunited with his wife, Yvonne, but, for security reasons, he found himself a different place to live and work. Grover-Williams' work name with SOE was "Sebastian." His task for SOE was to create a "sleeper" cell called
Chestnut which would prepare to take action against the German occupiers when ordered to do so by SOE. SOE's network in Paris was
Autogyro, headed by
Pierre de Vomécourt, but in April 1942 the Germans were destroying Autogyro and killing or imprisoning its members. SOE envisioned Chestnut as a replacement for Autogyro, but with the creation of the
Prosper network later in 1942, the much smaller Chestnut network became a secondary network in Paris, devoting itself to storing arms for the eventual use of the French resistance against the German occupiers. Grover-Williams recruited two fellow race-car drivers,
Robert Benoist and
Jean-Pierre Wimille, into his network and the trio used their wives and other women as couriers. The base of their operations was the Benoist family estate near
Auffargis about southwest of
Paris. Grover-Williams' main problem was that SOE did not supply him with a radio operator for communications with SOE headquarters in London. This problem was partially solved in January 1943 when Grover-Williams established contact with
Jack Agazarian, the newly arrived radio operator for the Prosper Network. SOE policy dictated that each network should have its own radio operator and that contact between and among networks was forbidden. Contrary to that policy, Agazarian transmitted Chestnut's messages to London. Starting in mid-February, Grover-Williams organized with SOE six parachute drops of weapons and other supplies for the
French resistance. In March 1943, SOE finally sent Grover-Williams a radio operator of his own, Roland Dowlen. The arms and other supplies the Chestnut network had received were stored for future use. Grover-Williams was also able to organize an effective sabotage effort at the
Citroën factory in Paris. Grover-Williams' downfall began 31 July 1943 when a German direction-finding van pinpointed the location of Dowlen and his radio and arrested him. On 1 August, Maurice Benoist, brother of Robert, was arrested. The next day Maurice Benoist led the German
Sicherheitsdienst (SD) to the Benoist chateau at
Auffargis. Grover-Williams was found hiding in a stable and arrested. The Germans recovered many containers of arms and equipment at the chateau. The Germans captured Robert Benoist on 4 August but he escaped, although he was later recaptured and executed. Jean-Pierre Wimille, the third of the race-car drivers who were SOE agents, survived the war. When Grover-William's wife, Yvonne, learned of his arrest, she fled Paris and took refuge with friends in the village of
Thorenc. ==Imprisonment and death==