A former MI5 spy is found dead from a shotgun wound by a terrified maid who screams when she sees him lying in a pool of blood at his luxury London flat. A partially blind widow is struck down by a taxi cab outside her Kensington home. She was holding an umbrella at the time and only a couple of months shy of her 70th birthday.

A young RAF pilot is shot down and dies crashing into the sea, his body is never recovered. Three years later his twin sister dies, from polio, just a year after getting married.

It's hard to imagine a family so beset with drama and tragedy but this is the story of Helen Melville Smith, known as Mel, and her family's tragic legacy, which began with the deadliest peacetime maritime disaster in history.

Smith lost her mother, husband, and both her twins before she was 49, but not before she had lost her beloved father as a child. At the age of just 14 she witnessed her first family tragedy of an unimaginable scale.

Like hundreds of others she stood on the dock in Southampton desperate for news after the Titanic, the world's biggest ocean-going liner, had hit an iceberg and sunk on her maiden voyage. Not only was Mel's father on board at the time, he was in fact the ship's captain, and ultimately responsible for the 1,517 souls lost at sea.

Captain Edward Smith became famous, or rather infamous, for his role at the helm of the ill-fated Titanic, the disastrous finale to his previously successful career at sea.

The British luxury passenger liner sank on April 14, 1912, en-route to New York City from Southampton, resulting in the deaths of hundreds of passengers and crew, including Smith - who is thought to have either shot himself or gone down with the ship.

Only 706 people survived. Since then countless books and films have been made about the tragedy seeking to unpick its many myths and questions. But now a new book charts the legacy of the family Smith left behind, in particular his only child, daughter.

Titanic Legacy: The Captain, The Daughter and the Spy, by Dan E Parkes is as much a family memoir as it is another book about the Titanic.

It delves into the life of the Captain's widow Eleanor, before she is run over and killed in the street, the couple's only child Mel, and the fascinating riddle surrounding the death of his son-in-law Sidney Russell-Cooke, a wealthy stockbroker, spy and Cambridge graduate who appears to have had a relationship with the economist Maynard Keynes.

Through never before published private letters and family photographs the book also tells how his inspirational daughter stepped out of the shadow of misfortune to carve her own path. Mel Smith died in 1973, aged 75, an only child, so Captain Smith's lineage ended on that day but not before she had led a full and fascinating life.

Author Dan Parkes insists: "Mel's 75 years were not simply punctuated by tragedy. She led a full life, growing up with a loving and attentive father, becoming a private pilot, a driver of fast cars, an artist's muse and collector of fine art, and living within a 'family' of suffragettes, politicians, royalty, Russian spies, yachtsmen, gay lovers and the spectacularly wealthy.

"This is the intriguing story of a legendary captain, who ironically once said he was 'Not very good material for a story', and of a daughter who stepped out of the shadow of misfortune to carve her own path."

Helen Melville Smith was born April 2, 1898, at 20 Alexandra Road, Waterloo, near Liverpool, the daughter of master mariner Edward John Smith and Sarah Eleanor Pennington. She had just passed her 14th birthday when the Titanic set off and would never see her father again. His body was never recovered and, worse still, she and her mother Eleanor had to endure Smith becoming the focus of blame for the disaster.

"Within days of arrival in New York, the survivors of the Titanic sinking were already criticising Captain Smith. Women who lost their husbands saw the Captain as responsible for their loss by not allowing their husbands into lifeboats," writes Parkes.

"Second-class passenger Mrs Amin Jerwan was reported as saying on 19 April 1912, 'Everybody on the ship blamed the Captain. The sailor who rowed our boat told me that he had followed the sea for 45 years and had never been in any kind of accident before, except on the Olympic when she rammed into the Hawke. That was under the same Captain'."

Under the headline 'Sympathy for Captain's Family', the Philadelphia Press of April 19, 1912, wrote: "There is no more pathetic figure in Southampton than Mrs Edward J Smith, widow of the commander of the ill-fated Titanic. The Smiths and their one daughter, a golden-haired and hitherto vivacious girl of thirteen years [sic], are absolutely prostrated. They refuse to see any but intimate friends."

The grief-stricken widow posted a message outside the White Star Line offices in Southampton that was subsequently reprinted by local and national newspapers under the heading 'Touching Message from Captain's Wife'.

Eleanor wrote: "To my poor fellow sufferers. My heart overflows with grief for you all and is laden with sorrow that you are weighed down with this terrible burden that has been thrust upon us. May God be with us and comfort us all. Yours in deepest sympathy, Eleanor Smith."

She paid tribute to her lost husband by initiating a yearly tradition: on St George's day she would send the Mayor of Southampton a 'buttonhole of red and white roses' in his memory.

Her only daughter Mel who, according to family letters published in the book, appears to have dealt with her father's death more easily than her mother, went on to marry wealthy stockbroker Sidney Russell-Cooke and have twins Simon and Priscilla, the grand-children the ill-fated Captain Smith was never to know.

Mel married Cooke in Mayfair in 1822.

By all accounts theirs was a happy marriage but she was in hospital recovering from a minor operation when he died of a gunshot wound on July 3, 1930. He was known to be suffering shell-shock as it was then called following his time fighting in France during the First World War and had suffered a nervous breakdown.

Various theories were put forward at his inquest as to whether he intended to shoot himself or was simply cleaning his gun with the safety catch off when it went off, shooting him in the stomach. Intriguingly, Cooke had met for lunch with his former lover, the economist Maynard Keynes, just before his death. The pair had been at Cambridge together.

Fellow biographer Gary Cooper, who wrote Life and Times of Captain Edward J Smith and the foreword for the revelatory new book, commented: "One has to wonder when reading the account just how much Mel knew about her husband's affairs, and what she thought of his curious, untimely death, which excited such a great deal of speculation at the time."

There is, of course, another possibility that the Russians killed Cooke, the book speculates. Unknown to most, possibly even his wife, Cooke was an MI5 spy, described as a "prototype James Bond".

But only a year after losing her husband in such horrific and mysterious circumstances Mel lost her mother. The partially blind Eleanor was run over and killed by taxi outside her Kensington home in 1931.

By the time the Second World War broke out, Mel's son Simon was an officer in the RAF. On March 23, 1944, the 20-year-old Flying Officer was on a mission to attack a shipping convoy off Bremangerlandet, Norway.

He came under fire, an engine was hit and the aircraft crashed into the sea.

"Mel would of course have held out hope that he had somehow survived and would return. But like his grandfather in 1912, his body was lost to the ocean and never recovered," writes Parkes.

Still reeling from the deaths of her husband, mother and son, three years later Mel was to lose her only remaining child. After only two years of marriage, Priscilla died on October 7, 1947, at Galashiels in Scotland, after a "short illness", aged just 24. The illness was later confirmed to be acute paralytic polio.

Somehow, despite all these unimaginable tragedies, Mel managed to survive and thrive.

"She was the last survivor of the Smith line, but she did not retreat from the world, or let her life be measured by the sorrows she had endured," says biographer Gary Cooper.

"She downsized and plunged back in, hosting parties at her new home and indulging in her many and varied passions, while in the 1950s when interest in the Titanic was rekindled, she played her part in adding gloss to the story.

"Indeed, Mel's tale brings the Titanic's story full circle, from the sad days following the disaster, through the lean years when people just tried to forget and forgive, to that point in the 1950s when she put pen to paper and recalled what she could of her father for Walter Lord, author of A Night to Remember, the book that arguably heralded Titanic's passage into popular culture."

More books would follow, films were made, and the story of the Titanic would enter popular culture. Captain Smith's own story, and that of his tragic family, would generally be subsumed... until now.

* Titanic Legacy: The Captain, The Daughter and The Spy, by Dan E Parkes with a foreword by Gary Cooper (Amberley Publishing £25) is out now.