Sheltering in a sunken road near the French village of Beaumont Hamel on 1 July 1916, Captain Edmond McNaghten "Pongo" Dawson believed the first British thrust during the Battle of the Somme would be swift and decisive.
The German lines had been subjected to heavy bombardment for an entire week, and the Allies had the advantage of vastly superior numbers.
As the order came just after dawn to send the troops over the top, Captain Dawson was captured on film ushering his men, of the 1st Lancashire Fusiliers, along the trench. As commander of the company, he was one of the first on to the parapet. A few seconds later, he was also one of the first to be cut down by German machine-gun fire.
For decades, historians have argued over the veracity of the film shot that morning, the first day of the Battle of the Somme. Certainly, some scenes were re-enacted and filmed for propaganda purposes. But now, using a series of new scientific techniques, analysts have proved for the first time that most of the images are genuine, enabling them to identify many of the combatants and trace their surviving relatives.
One descendant who got to "meet" her grandfather, Captain Dawson, thanks to the documentary-makers was Anne Dawson.
The young captain, so fearlessly preparing to lead the advance - filmed that morning by the celebrated cinematographer Geoffrey Malins - miraculously survived the multiple gunshot wounds he sustained in the assault and was invalided back to Britain. He died in the mid-1960s, when Ms Dawson was still a small child.
"I knew that my grandfather had been injured at the Somme, but to actually, physically, see him there was amazing," said Ms Dawson, a former ITV newsreader and mother of two from Cheltenham, Gloucestershire.
"I was just fascinated to see him, and it was great for my daughters to be able to see their great-grandfather too. When he looks towards the camera, the family resemblance is incredible: he has a jawline and an expression on his face which look just like my father and my brother.
"I remember my grandfather as an avuncular, bald-headed man who sat me on his knee and played the harmonica. To see him so much younger and about to go into that horrible carnage was a very powerful experience. You can hear stories about relatives, but actually seeing him there made it absolutely real. It was very moving; the hairs on the back of my neck stood up and for a brief moment I was in that picture with him."
The first day of the Battle of the Somme was the bloodiest 24 hours in the history of the British Empire. By the next morning, nearly 60,000 young men - a large number of them civilian volunteers - would be lying dead or wounded on the killing fields of north-western France.
The Allied forces were attempting to break through German lines along a 25-mile front - both north and south of the River Somme. But far from the swift, decisive master-stroke that most were expecting, the battle swiftly descended into a hideous war of attrition, fought in the stinking, blood-spattered trenches of the western front.
The carnage has gone down in historical infamy, not least because of Malins's shocking footage - arguably the first film of battle in British military history.
When Malins's footage was released in British cinemas the following month under the simple title The Battle of the Somme, it was a sensation. The images of soldiers going over the top and into no-man's land, advancing on German positions under heavy machine-gun fire,and carrying back mortally wounded comrades, was nothing short of the UK's first box-office smash. The film attracted a domestic audience of 20 million - a figure not matched until the release of Star Wars in 1977, and accounting for nearly half the entire population of wartime Britain.
Now, on the eve of the 90th anniversary of the Somme, a team of documentary-makers and historical experts, including specialists from the Imperial War Museum, National Army Museum and Scotland Yard, has for the first time subjected the film to critical scientific analysis.
The team used techniques ranging from satellite analysis of trench locations to professional lip-reading and the latest facial-recognition software to prove the genuine nature of nearly all the battle scenes in the film. They also identified at least one "fake" attack, edited together with the original footage, filmed in a shallow trench with soldiers wearing incorrect uniforms and equipment.
The investigators, whose work will be aired in a documentary on Five this coming Saturday, also managed to identify for the first time some of the British soldiers fighting and dying in the combat footage, the earliest example of such scenes being captured on film.
Among these was Captain Dawson, whose family previously had no idea that there was footage of him that crucial morning.
Andy Robertshaw of the National Army Museum in Chelsea, west London, said the work, to be broadcast in Battle of the Somme: The True Story, was a "significant" step towards a greater understanding of the iconic battle.
"These techniques give an amazing immediacy to the original footage - particularly with things like the lip-reading," said Mr Robertshaw. "It gives the Battle of the Somme a sense of reality - to go from grainy footage of vague people in a silent film to 'this is your grandfather; nice to meet you'. This is pretty much the closest you can get to the First World War without a time machine."
The facial recognition software, more normally used by Scotland Yard to identify criminals, also came up with a positive match for another soldier in Malins's original footage. William Holland, who served with the Royal Engineers during the First Wold War, is seen carrying a wounded colleague towards the camera.
Private Holland's son, 78-year-old Maurice, joked: "It is my dad. He wasn't a hero though; he was probably in a trench puffing on a Woodbine and somebody said: 'Can you carry this man?'"
Malins's original film tells a darker story. Shortly after this scene, a caption informs the audience that the injured man died just 20 minutes later.
Roger Smither, head of the film and video archive at the Imperial War Museum, which owns the copyright to The Battle of the Somme, said: "The film created a very, very big noise when it was first released in 1916, and there was a certain amount of discussion over its controversial nature.
"It gave people in Britain the feeling that they were sharing the experience of the soldiers actually on the front line, and had a major impact on the national consciousness at the time. This recent work extends that sense of recognition and identification, 90 years on."
Zero hour for the Battle of the Somme was 07:30 on 1 July 1916. At that moment, for entire battalions of young men like Captain Dawson, there was a brief and unsettling silence as the artillery shifted their aim on to the agreed line of targets.
Then, in the words of the poet John Masefield: "All along that old front line of the English there came a whistling and a crying. The men of the first wave climbed up the parapets, in tumult, darkness and the presence of death, and having done with all pleasant things, advanced across no-man's land to begin the Battle of the Somme."
The first day of the battle, codenamed Z-Day, was generally accepted to be the worst of them all, with some battalions suffering losses of more than 90 per cent.
The Battle of the Somme was supposed to be won by the Allies on that first day of July. It was partly thanks to this overconfidence that the generals allowed Malins access to the trenches. Instead, the battle lasted until November - long after the finished film had been screened at home. By the end of the offensive, there were more than one million casualties from both sides. After five months of bitter fighting, the Allies had advanced just five miles.
As horrific as the battle was for the British troops who suffered and died there, it cost hundreds of thousands of French and German lives as well. One German officer famously described the Somme as "the muddy grave of the German field army".
Among those to experience the horrors of the battle from within the trenches were a young JRR Tolkien, later to write the epic Lord of the Rings, the poets Robert Graves and Siegfried Sassoon, future British Prime Minister Anthony Eden - and an Austrian corporal named Adolf Hitler.
This month, a number of vivid letters and diary entries penned in the trenches on the first day of the battle were unearthed from the Imperial War Museum's collection and placed in a permanent online exhibition.
The originals of the letters and diaries are on show in an exhibition at the museum, alongside items from the battle including a revolver carried by Tolkien and a football kicked across no-man's land by one battalion as they advanced on Z-Day. Among other items on show is a draft of one of Graves's poems with alterations by Sassoon.
The 90th anniversary of the start of the Battle of the Somme will be officially marked on Saturday with a commemorative service in France, attended by the Prince of Wales and the Duchess of Cornwall, as well as 110-year-old Henry Allingham, the last surviving British veteran of the Somme.
'Battle of the Somme: The True Story' will be shown on Channel Five at 11.45am on SaturdayReuse content