As the old patriotic song says, "There'll always be an England ..." and Britain's history remains irresistible to filmmakers and audiences alike. Proof of that is reflected in the television series Downton Abbey.

The show's vast international audience could not get enough of the pomp and circumstance of times gone by. With opulent costumes and perfectly judged performances from Dame Maggie Smith, Hugh Bonneville and Elizabeth McGovern, it offered romance and drama.

The show, however, is neither a witty Oscar Wilde comedy nor a soap opera in vintage frocks. It's a sharply focused account of a complex social and cultural turning-point in British history. The story starts in 1912, when the clouds of World War 1 were already gathering. But the wealthy families in their grand houses, like Downton Abbey, scarcely paid any attention.

Historians have dubbed this period "the Edwardian twilight", because it signified the end of an aristocratic way of life that had held sway since the reign of ElizabethI. The archaic class system was still firmly in place.

But this television series explores why that fragile facade collapsed when confronted by the stark realities of World War1.

The main narrative features Robert, Earl of Grantham (Hugh Bonneville), who realised that his family's opulent lifestyle had sucked up all their money. To survive, he made a "good" marriage, that was also a solid commercial merger, to Cora (Elizabeth McGovern), a rich American heiress, who brought a substantial dowry that restored the family's fortunes.

Fate, however, dealt the Grantham family a terrible blow with the untimely death of the their male heirs (by law it could not be left to any of the Grantham women). A legal investigation found the next heir, who would inherit the grand estate, its contents and all the family's money: a distant cousin, Matthew Crawley (Dan Stevens), a young solicitor.

Crawley's arrival at Downton Abbey with his mother, Isobel (Penelope Wilton), is like two worlds colliding. Crawley actually works for his living, a concept alien to the Grantham clan. His presence reveals the cracks in the opulent facade. His mother is determined to stake her claim in the family, but that produces an immediate feud between Isobel and Violet, the Dowager Countess of Grantham, played with ferocious arrogance by Maggie Smith.

Matthew is instantly attracted to the daughter Mary (Michelle Dockery) but she rebuffs him and, to make things worse, a further shock comes when Crawley decides that he sees no reason why he should not continue his career as a solicitor, instead of just being an idle "lord of the Manor".

The nimble structure of the script also explores the servants of Downton Abbey, who are as prominent and involved in the story as their employers. They maintain the house but "below stairs" they know everything about the family, and they have their own personal grudges.

There are intrigues, blackmail and even a touch of "the love that dare not speak its name" and the real fascination is that neither the masters nor the servants realise that their world is undergoing vast changes.

Karl Marx and Friedrich Engels were calling for a classless, stateless society and their doctrine was filtering through Europe. Their world was rapidly changing and in the latter part of the story, in 1914, an anarchist assassinated Archduke Franz Ferdinand in Sarajevo, igniting a conflict that would eventually draw the whole of Europe into World War1.

That's the backdrop to the story that author Julian Fellowes wrote. He has a title as long as your arm - Julian Alexander Kitchener-Fellowes, Baron Fellowes of West Stafford - and he's also the only Oscar-winner in the history of the awards to have a hereditary title.

Fellowes lives in Los Angeles and has acted in several films, but his real skill is writing adaptations for classic movies such as The Importance of Being Earnest (2002), Vanity Fair (2004) and the charming The Young Victoria (2009). The script that totally changed his life, however, is Downton Abbey.

Fellowes describes himself as "the proverbial outsider", a bit like the character of Matthew Crawley. He was sent to élite institutions such as Ampleforth and Cambridge, but he never really fitted in.

"I was always 'the bottom of the top'. I wasn't handsome, titled or rich," Fellowes said. "I was only asked to join my schoolmates because someone else had dropped out. Nonetheless, that taught me to be a sort of fly on the wall. Nobody pays any attention to you, but you observe everything," he said.

"My colleagues describe me as a chronicler of the British class system and I am proud to think the thing that we got exactly right with Downton was that we presented the characters of the servants and the family in exactly the same way. Some of them are nice, some of them are not nice, some of them are funny, some of them are not, but there is no division between the way the servants and the family are portrayed.

"It would have been an error to represent the servants as writhing in a state of permanent torment, while everyone upstairs was vicious and violent, horrible and dishonest," said Fellowes. "That allows the characters no space for personal change or insight. My vision is that both groups are just people trying to bash through their lives as best they can, and that makes their performances honest."

When asked how he felt about the show's success he said: "I think I'm more fearful of the future now. I always feel that there's some giant hand about to lean in and snatch it all away from me, telling me: 'That wasn't meant for you.' I know it's absurd to live your life dreading some unspecified disaster, but lifelong habits are hard to break.

"There is always a lingering ghost of you if things don't work out, a sad figure trying and trying to get it right, but, in this case, it did work and that sad figure has now receded, to a degree.

• "Downton Abbey screens on BBC Entertainment, DStv Channel 120, tonight.