The images shown here tell the story of a love affair between young people from opposite ends of the earth, brought together by war and finally separated by peace.

Old glass photographic plates bearing these pictures, lost for almost a century, have surfaced in northern France. They were not meant to be seen by you or me. They capture a moment of intimate, jesting relaxation on a sunny day in a French garden 97 years ago, a few miles from the most calamitous of all the battles of the First World War.

Look carefully at the image of the young officer wearing a New Zealand uniform and distinctive lemon-squeezer hat who is saluting the camera, probably in October 1916. Look at the two other images of "him" with another New Zealand officer who has a moustache.

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Now look at the fourth picture, which shows a woman in her late twenties or early thirties sitting, in an affectionate, almost raunchy pose, on the moustachioed officer's outstretched knee.

If you examine the images carefully, you will see that the "officer in the hat" and the young woman on the officer's knee are the same person. All four images were taken within a few minutes of one another, in the garden of a substantial house 30 miles behind the battle front of the Somme.

The positions of the leaves and flowers in the garden and the angle of the shutters on the house behind are identical. Between the shots, the two young people have scrambled to change their clothes. She has put on his first lieutenant's tunic, his New Zealand Division "lemon-squeezer" hat, his baggy officer's trousers, his belts, his puttees and even his spurs. She is still wearing her floppy white blouse, stuffed down under the tunic. She has an engagement or wedding ring on her left hand.

He is also wearing an officer's tunic but he has changed into a rough pair of trousers. He can also be seen in the photos to the left and inset. The first was probably taken in the spring of 1916 when he was still a second lieutenant. The other was taken two years later, in the summer of 1918, when he has been promoted to captain and is wearing the riband of the Military Cross.

Thanks to painstaking detective-work by one of the most eminent New Zealand historians of the First World War, Andrew Macdonald, the identity of the officer in the moustache has been pinpointed for The Independent with a high degree of certainty. The woman's identity is still unknown but Macdonald's researches, and our reasonable deductions, suggest that she is a local French woman with whom the officer had a close friendship, to say the least, between the spring of 1916 and the summer of 1918.

The building seen in the background of the garden shots has been identified by a local historian and blogger, Frédéric Bellegeulle, as the "Villa des Acacias" on the Rue Saint Denis in Hallencourt, near Abbeville.

Captain Albert Arthur Chapman, born in Tasmania, Australia, in March 1880, and therefore 36 to 38 at the time of the photographs, returned to New Zealand unmarried. Was the woman on his knee a friend; or his fiancée; or the wife, or widow, of a French soldier? The intimacy, and cheekiness, of the images suggest that she was more than just a friend.

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Since 2009, The Independent has been publishing a series of "lost images" of British and Commonwealth soldiers of the First World War, taken from glass photographic plates which have come to light in attics or barns or rubbish skips in the Somme. The images were, we believe, taken by local amateur photographers. Copies were given or sold to soldiers as souvenirs or to send to their loved ones. The historical value of the plates, as a record of the British and Empire armies during the most murderous battle in our history, was first grasped by two local men: Bernard Gardin, a photographer, and Dominique Zanardi, proprietor of the "Tommy" café in Pozières in the heart of the Somme battlefield.

Two more batches have now reached Mr Zanardi and have been processed by Mr Gardin. A batch of 21 plates, found in Hallencourt, 30 miles east of the battlefields of 1916, includes the extraordinary sequence of six images shown here.

It was Mr Zanardi who first realised that the "officer in the hat" was also the "woman on the knee". He thought that he might have come across a rare picture of a female officer. This proved to be impossible: neither the British nor the New Zealand armies had women in military uniform at this period.

We showed the images to Macdonald, a New Zealand military historian living in London, and the author of On My Way to the Somme: New Zealanders and the Bloody Offensive of 1916 and Passchendaele: The Anatomy of a Tragedy (both HarperCollins).

He identified the officer in the moustache from his collar and cap badges as a member of the 7th Southland Mounted Rifles, one of the squadrons in the Otago Mounted Rifles (not cavalry but troops who could fight on foot or on horseback). In the portrait photograph with other soldiers, he is wearing the collar badges of the New Zealand Pioneer (Maori) Battalion (to which he has been transferred). In the image of him sitting backwards on a chair, he has been promoted to captain and wears the riband of the Military Cross.

"In the records of the New Zealand Pioneer (Maori) Battalion there are only two officers who meet these criteria. One of these did not reach the rank of captain, leaving Albert Arthur Chapman as the prime candidate," Macdonald said.

"His service record fits the man in these images like a glove. He was transferred from the Pioneer Battalion to serve with the New Zealand division headquarter staff behind the lines of the Somme in April, May and June 1916. He was transferred back to the same Pioneer battalion when the New Zealand Division entered the battle [from 15 September]. He was promoted to first lieutenant in October 1916. He won the Military Cross in June 1918."

The record shows that Captain Chapman was serving in New Zealand divisional headquarters in the spring and early summer of 1916, close to Hallencourt, where the images were taken. He probably met the "woman in the hat" at that time.

The rest of the story can be pieced together from intelligent guesswork. The images in the garden cannot have been taken before October 1916, after Chapman was promoted to first lieutenant. The summer/early autumn feel of the garden suggests that it might have been that month. The image of Chapman as a captain and wearing the Military Cross riband cannot have been taken before June 1918. He must have gone back to Hallencourt – presumably to visit the woman. All the glass plates were found together and were, therefore, almost certainly taken by the same photographer.

We can deduce, therefore, that Chapman knew the woman for around two years. He went back to visit her at least twice, which suggests a deep friendship or an even more intimate relationship. She seems to be a confident, intelligent, rather modern-looking woman for her time. Is the ring on her finger Chapman's engagement ring or her wedding ring? Had her husband died in the mass slaughter of the French army in 1914-15? Or – less flatteringly – was he away fighting in the battle of Verdun (February to November 1916)?

The New Zealand service records consulted by Andrew Macdonald show that Chapman was born in Tasmania on 4 March 1880. Before the war he was a clerk for a shipping company in Dunedin, New Zealand. He volunteered as a trooper, or private, in October 1914. He probably served at Gallipoli and was promoted first to sergeant then to second lieutenant.

Chapman returned to New Zealand in October 1918, just before the war ended. He was unmarried when he left and unmarried when he returned. There is no trace of him marrying before he emigrated to the United States, via Canada, in 1924. So what happened to the "woman in the hat"? Did he jilt her? Did she jilt him? Did her husband return from the war?

Maybe – and this is speculation – she died in the global flu pandemic of 1918, which killed more than 400,000 people in France and took a particularly heavy toll in the war-ravaged north. The population of Hallencourt, nearly 2,000 in 1911, had fallen by one-sixth by the end of the war.

This would be the ironic novelist's or movie-maker's ending to the love story implied by these images. Our soldier survived four years of the appalling conflict, the centenary of which will be commemorated next year. Perhaps, his French civilian sweetheart did not. Does that explain why Captain Chapman, a handsome, decorated war hero at a time when men were scarce, never married?