She has a million-dollar smile, and as a gentle Hebridean breeze ruffles her hair, young Mary Anne MacLeod grasps the handlebars of a motorcycle looking every inch a young lady set to go places.

The previously unpublished picture was taken in the small Lewis village of Tong sometime in the 1930s. Before long, Mary Anne would be off on a life-changing journey, one which would eventually touch millions of lives and lead to the door of the 45th President of the United States of America.

Now a new documentary that traces Donald Trump’s mother’s route from humble beginnings in the Western Isles to domestic service in New York – and, of course, to her famous son – is set to shed fascinating light on her character and the impact she had on those who knew her.

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The President’s Mother, due to be screened on BBC Alba on Tuesday, also provides a glimpse into the hopes and dreams of a young Western Isles teenager shared in intimate correspondence with the Dundee pen pal she met just a handful of times, but who would hold a special place in her heart for 60 years.

While Mary Anne MacLeod would go on to marry builder Fred Trump in New York, and her son go on to become a US President, Dundee-born Alice Stiven, her teenage pen-pal, would set off on an adventure of her own which would take her to the heart of Nazi Germany and a frightening escape to freedom.

The pair lost touch for decades, only to meet again by chance.

Their deep bond is now being turned into an illustrated novel by Alice’s grand-daughter, Cathy Brett, in a touching tribute to their lifelong connection.

The BBC Alba documentary criss-crosses the Atlantic, from the tiny community of Tong where Mary Anne was born in a traditional blackhouse to the towering skyscrapers of New York. It draws on touching correspondence to shed light on the hopes and dreams of two young Scots women as the country tottered between two world wars and the hardships of the Great Depression.

In one letter, penned when she was in her mid-teens, the young Mary Anne laments living in peaceful Outer Hebrides, telling her new friend of how she was writing “from the lonely Isle of Lewis” and hinting at the adventurous spirit that would eventually see her leave Glasgow on board the SS Transylvania for a new life in New York.

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The east coast of America would seem a world away from Lewis in the 1930s – which was still recovering from disputes with wealthy landowner Lord Levenhulme that had resulted in bitter land raids and unemployment – while work prospects for young women were limited to working in fish factories and gathering seaweed.

Meanwhile, Alice, the same age as her young friend, reveals her own anxieties; acutely clever yet less well-off than other students at Dundee High, she later told in her memoirs of going to school conscious that her clothes had been sewn by her mother. “I felt myself to be a second-class pupil,” she wrote.

According to Ms Brett, the two teenagers exchanged letters for several years before finally meeting for the first time in Glasgow shortly before both would embark on new lives in other countries.

The decisions they took to leave home would have life-changing consequences for both: Mary Anne travelled to New York to work in domestic servicem which led her to meet her future husband, Fred Trump, the son of a German immigrant, while Alice opted to pursue her studies in Germany in 1933, unaware that war was about to tear Europe apart.

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She married a man who would go on to become an army officer, eventually fleeing to safety with American troops who found her cowering in her home with her two young children.

“Mary Anne made a big impression on my grandmother and they clearly enjoyed writing to each other,” says Ms Brett. “They seem to have had a lot in common, both were ambitious to get away from home and to see the world.

“I think they were both quite feisty. Both made the decision to go somewhere completely new which must have been difficult.”

With both making their way in the world in new countries, they lost touch for decades. It was only during an infamous television interview with broadcaster Selina Scott when Mr Trump introduced his mother, that Ms Brett’s astonished grandmother realised the glamorous woman on her television screen was her long lost pen pal.

They later met in luxurious surroundings of London’s Dorchester Hotel.

“Mary Anne told her that she had lost hope of ever seeing her again,” recalls Ms Brett.

The documentary, presented by journalist Torcuil Crichton, includes interviews with residents of Tong who recall Mary Anne returning from her New York home with stories of glittering parties and her glamorous lifestyle.

However, according to producer Calum Angus Mackay, she appeared to remain unaffected and even retained her gentle Lewis lilt.

“She is very much a background figure in Donald Trump’s life and there’s not an awful lot known about her personality,” he says.

“There’s a curiosity about Donald Trump and whether there is any part of his DNA that has an element of island compassion or sentiment.

“We find out that Mary Anne had something of a double life, there was a good number of times that she came home to Scotland and people speak of how she may have looked like she would have airs and graces but she was really very down to earth.

“During her visits home, she went to church and conversed in Gaelic.”

President Trump paid a flying visit to Lewis in 2008.

He spent just 95 seconds inside the pebble-dashed croft house where his mother grew up and delivered gifts to family members, thought to be autographed copies of his own book, How To Get Rich.

The President’s Mother, The Mary Anne MacLeod Story, will be shown on BBC Alba on Tuesday at 9pm.