The secret plan for 'D-Day Two' if Normandy landings had failed: SAS team led by one-armed Frenchman parachuted into Nazi-held Brittany night before June 6, 1944 to seize port and enable Allies to invade, reveals historian DAMIEN LEWIS

Two black Citroen saloon cars – typical Gestapo vehicles – crawled through the narrow Brittany lanes, the drivers clearly lost. 

Moment later their occupants – all Gestapo agents – were dead or running for their lives.

A flaming projectile tore apart the first vehicle, turning it into a ball of searing fire, and bullets raked the second. 

Four men bailed out and ran. Three were captured by their assailants, but the last managed to slip away.

This did not bode well for the ambushers – the men of the SAS, who here in Brittany were fighting alongside their French Resistance comrades. 

These 36 SAS had been the first Allied troops to land on French soil, as the vanguard for Operation Overlord they had parachuted behind the lines into Brittany. 

Landing at 23:40 hours on the night of the June 5, 1944, they hit the ground before the main Allied troops made the Normandy beaches.

Theirs was a top secret and vital mission.

Led by the one-armed Pierre-Louis Bourgoin, a French SAS colonel, and a former British Secret Intelligence Service agent, they were charged with 'attacking railways, cutting telecoms, striking opportunity targets e.g. attacks on soft vehicles, sentries...'

Their aim was to cut off Brittany from the Normandy Beaches, so preventing the mass of German troops stationed there from attacking the Allied forces spearheading the D-Day landings.

But the SAS also had another, ultra-secret role. They were to gather intelligence on the port of Saint-Malo, on the northern Brittany coastline, and to secure maps and plans of the enemy's defences. 

A second D-Day landing operation was being planned, one that would involve sailing British and American warships directly into Saint-Malo, to land troops. 

If that could be done, the Allies would have outflanked the enemy, easing pressure on the Normandy beachheads.

By the time the entire SAS force had been dropped in, Bourgoin commanded some 180 men at arms. 

But with the best will in the world, that size of force could never take Saint-Malo port and hold it for long enough for the Allied ships to arrive safely. 

Hence Bourgoin's other key mission - calling in mass airdrops of weaponry and explosives, to arm the thousands of Frenchmen flocking to join the Resistance.

After rudimentary training furnished by Bourgoin and his men, the SAS and Resistance were to combine forces to wrestle Saint-Malo from enemy hands. 

Still, it was a tall order. To help Bourgoin and his men pull it off, a dozen 4x4 Willys jeeps bristling with rapid-firing Vickers-K machine-guns had also been parachuted in.

Each jeep had been winched up into the open bomb-bay of a Halifax bomber, with a giant chute lashed to each of its four corners.

Bourgoin had lost his arm while serving with the Secret Intelligence Service, in North Africa, in 1943. 

His vehicle had been attacked by a German warplane, causing him multiple injuries. 

Undaunted, he'd gone on to volunteer for the SAS, and was now back in his native France determined to achieve his mission.

All had been going well, until that Gestapo agent escaped from the ambush, raising the alarm in the nearby town.

As Bourgoin fully appreciated, the Gestapo escapee had seen SAS soldiers attacking his convoy. The enemy were bound to come hunting. 

Well-armed with scores of Bren machineguns and US-made Bazookas, Bourgoin set his defences. At dawn the following day the enemy came. 

They chose to attack across a thick cornfield. As the grey-uniformed troops closed in, they left furrows in the crops, which meant they could be seen and targeted. 

Caught in the open by Bourgoin and his men, very quickly heaps of bloodied enemy corpses lay amongst the golden corn.

But still the enemy kept coming. Bourgoin's men fell back from the farmstead they'd been defending, at the village of Saint-Marcel, to their nearby headquarters at the Chateau Saint-Genevieve. 

Their wounded were loaded aboard jeeps, and rushed to a makeshift Resistance hospital. But four jeeps were kept for the fight. 

Racing ahead, they outflanked the advancing enemy, and began to pour in fusillades of fire from their pivot-mounted Vickers-K machine-guns. 

Scavenged from obsolete RAF warplanes, the rapid-firing Vickers-K made a perfect weapon for the kind of fast-moving, hit-and-run warfare of the SAS.

They proved equally lethal against the enemy assaulting the Chateau Saint-Genevieve. 

Hitting hard, then zooming away, to strike again from a new direction, the jeeps turned the tide of the battle, but not for long.

The enemy's response was to bring in reinforcements, including armoured cars and heavy guns. Bourgoin knew his men had to melt away, or all would be lost. 

He radioed SAS headquarters in London, even as the enemy broke through to the Chateau Saint-Genevieve, in fierce close-quarter fighting.

An SAS man on the chateau's roof wielding a Bren kept the enemy at bay. At one point he dropped a grenade down a chimney, when the enemy broke into a room below. 

Via his radio, Bourgoin got orders from London to disperse and save as many of his men as possible. 

Blowing up the massive ammo dump they had assembled from the supplies parachuted in, they began to slip away under cover of the smoke and explosions.

It was nigh-on midnight when the last of Bourgoin's fighters got away, pursued all the way by 'crack SS troops.' 

In making their escape, the SAS would be scattered to the four corners of northern Brittany. 

Worse still, their radios were either destroyed in the firefight or out of service. 

That night an entire SAS Squadron – some 180 men – would be listed as missing in action, for London had no idea if any had survived.

Brigadier Roderick McLeod, overall SAS commander at the time, responded by calling for volunteers. 

He needed a team to mount Operation Lost, a mission to go in and find the missing squadron, and to resurrect the plans for the second D-Day landings. 

The battle for Normandy was hanging in the balance, the enemy giving no quarter, and the Saint-Malo operation was looking increasingly likely.

The man who stepped forward to lead Operation Lost was the tall, ramrod straight figure of Major Oswald Cary-Elwes, a long-standing SAS commander who'd fought in North Africa and Italy. 

Blessed with a wholly unconventional mindset, Cary-Elwes was wont to seek out SAS recruits in the brig. 

In one instance, he'd recruited Corporal Charley Hackney while imprisoned and facing a court martial. 

Cary-Elwes judgement of the type of man who made an ideal SAS recruit was uncanny. Hackney had gone on to become an SAS stalwart.

Cary-Elwes parachuted into Brittany to find the missing SAS squadron, along with his ever-faithful batman, Corporal Eric Mills. 

Mills, blessed with the ability to 'laugh in the face of danger' doubled as Cary-Elwes bodyguard. As Mills avowed, the SAS major 'took some bloody guarding and all.' 

Cary-Elwes always had to have his cup of tea in the morning, though he never drank it, and he had the habit of strolling along lanes deep in enemy-occupied territory whistling his favourite English tunes at the top of his voice.

Cary-Elwes was also fluent in French, apparently fearless and with a daredevil attitude that a mission like Operation Lost called for. 

Having landed in France, the search for Bourgoin began. Unfortunately, the Gestapo had resorted to arresting all one-armed men in Brittany, in an attempt to nail the SAS colonel. 

This made Bourgoin decidedly hard to find. The Gestapo had also started to dress in the uniforms of those they had killed or captured, to masquerade as SAS and entrap their fellow soldiers.

It took all of Cary-Elwes' guile, audacity and front to winkle out Bourgoin. Once reunited, Cary-Elwes, who'd parachuted in with a radio set, was able to contact SAS headquarters with the good news. 

Bourgoin and most of his men were still at large. In the battle around Chateau Saint-Genevieve the SAS and Resistance had killed four hundred enemy troops. 

But they'd also suffered losses. Thirty SAS and Resistance fighters had been killed and many more wounded.

Worse still, those men who had been captured had been finished off by the Waffen SS with a bullet to the head. 

As Bourgoin explained, the 'whole area was full of Germans hunting for him and they had put a large price on his head.' 

Yet he and his men remained undaunted. With radio contact to London they could call in more weaponry, arm the Resistance flocking to the cause, and execute their mission.

Cary-Elwes and Bourgoin's orders from London were to give absolute priority to 'the operation for the capture of St Malo,' as well as securing intelligence on other key ports in Brittany. 

As Bourgoin was being so intensively hunted, Cary-Elwes and Mills joined a force led by his deputy, Lieutenant Marienne, known to all by now as 'The Lion of Saint-Marcel', after the epic battle they had fought in and around that village. 

Shortly, they hit the jackpot, being passed a thick wad of captured enemy documents and maps, detailing the key defences of Saint-Malo and the other main ports.

Such priceless intelligence couldn't be radioed back to London. Only by escaping back to Britain could Cary-Elwes get this into Allied planners' hands. 

He was ordered to do so at all possible speed. Via the Shelbourne Line – a long established escape line run by MI9, the British 'escape factory' – he and Mills were to be spirited back to British shores. 

Dressed as French farmers, with fake IDs, they were hustled through Brittany countryside crawling with the enemy, at first in a battered van, which for some reason tooted its horn whenever the brake was applied.

Transferred to a horse and cart, Cary-Elwes high spirits got the better of him. Passing a French village, he declared they should stop at the café for brandy. 

They were served fiery Brittany Calvados – a cider brandy – and Mills, unused to such strong liquor, choked himself half to death. 

From then on he vowed never to touch French alcohol, earning the nickname 'Le petit caporal qui ne boit que du lait' - the little corporal who only drinks milk.

They reached the last stop on the Shelborne Line, a remote house perched on the Brittany cliffs, with a Royal Navy Motor Torpedo Boat (MTB) scheduled to pluck them off the nearby beach the following night. But there disaster would strike. 

In their wake, Lieutenant Marienne, 'The Lion of Saint-Marcel', and his men had been surrounded and captured. 

Though Marienne had argued vehemently that they were bona fide Allied combatants serving in uniform, and should be treated as prisoners of war, they were tortured and shot.

Even as Cary-Elwes and Mills were so close to executing their getaway, so the hunters came. The enemy surrounded the remote farmhouse. 

Yelling drunkenly, they fired through the door and demanded the owners – a young Frenchman, his wife and child – open up. 

Cary-Elwes and Mills hid in the loft, vowing to use their grenades and pistol to make a last stand. 

But in the dark and confusion the drunken troops managed to shoot their own officer, and were forced to load him aboard a cart to rush him to hospital.

In that moment, Cary-Elwes, Mills and the French family slipped away. Hiding in a cornfield they watched as the enemy returned and put the farmstead to the torch.

It had been used to store arms for the Resistance. When the flames hit the explosives the enemy were caught in the blast, several being injured and killed. Hunted on all sides, the fugitives lay low. 

They made their way to Bonaparte Beach under cover of darkness the next evening.

Even now, menace stalked them on all sides. The path to the beach was marked by white handkerchiefs, placed there by a brave French girl. 

It was to show the safe route through a minefield. After a tense wait, a rowing boat emerged from the gloom, and they piled aboard. 

Shortly, they were hauled onto the waiting MTB and sped back to British shores at top speed.

Once safely ashore, Cary-Elwes delivered his intelligence into the eager hands of Allied commanders. 

While the second D-Day landings in Brittany were never actually needed – the breakout from the Normandy beachheads was soon underway – the plans and maps Cary-Elwes had delivered were used to guide Allied bombing missions of those key Brittany ports.

Two weeks after escaping to Britain, Cary-Elwes and Mills would parachute back into France, to link up with Bourgoin – the one-armed French Colonel – and his surviving SAS, to liberate Brittany from the Nazi occupiers.

SAS Great Escapes Three by Damien Lewis is published by Quercus at £22.