When Celia Sandys was born in 1943, her grandfather, Winston Churchill, was at the peak of his powers. He had been prime minister for three years. The Nazi invasion had been postponed indefinitely after the Battle of Britain, and Churchill had finally persuaded President Franklin Roosevelt to join the war. The overwhelming might of the American forces and materiel being assembled in Britain in preparation for D-Day the following year meant it was only a matter of time before Hitler would be defeated.

The young Celia, however, daughter of Churchill’s daughter Diana and the former British colonial secretary Duncan Sandys, spent her early life largely oblivious that she was the granddaughter of the savior of Western civilization. Bit by bit, she came to understand the importance of the man she knew simply as Grandpapa. Her personal memories form the heart of “Chasing Churchill,” a three-part series that starts tonight on PBS.

Ms. Sandys is full of telling stories about growing up with the 20th century’s greatest Englishman. “My grandfather sent me a toy bulldog with a note, ‘Just look after him. Your loving Grandpapa,'” she said during an interview in her Upper East Side apartment. “I asked why he had given it to me and was told it had been sent to him and he didn’t want it. I wondered why, of all things, my grandfather should have been given a toy dog. I was to discover, of course, that to everyone else he was ‘The Great British Bulldog.'”

As a child she often used to spend weekends with her grandparents at Chartwell, their country house in Kent. Then, as “an available granddaughter of an appropriate age,” she joined the peripatetic Churchill on a succession of painting and gambling vacations in Provence. She was with him in Monte Carlo in 1962 when he fell and broke his hip at the Hotel de Paris — and all those around him thought he would soon die. Few others were as close to him during his final years.

PBS’s “Chasing Churchill” is a fascinating and intimate portrait of the great man seen from within the family. Ms. Sandys gently leads viewers through Churchill’s life — from fighting in Cuba to incarceration by the Boers in Pretoria to discussing war strategy with Roosevelt in Marrakech.

Most interesting to Americans, perhaps, is Churchill’s lifelong love affair with America, the country where his mother, Jenny Jerome, was born. He learned his politics and trademark stentorian speaking style from the veteran New York Democratic politician Bourke Cockran. On the West Coast, he stayed with the newspaper and movie tycoon William Randolph Hearst in San Simeon. He visited Charles Chaplin on the set of “City Lights” — “Bolshie in politics; delightful in conversation” — and was nearly killed when run over by a cab in Manhattan.

She conveys her own experiences of her grandfather in a charming, unobtrusive manner. Her aristocratic family took his eminence for granted, and she had to glean from others the scale of his greatness. “You could not help but notice that people talked about him in a different way and behaved with him in a different way,” she said.

And with privilege came duties and responsibilities. “I remember feeling extremely embarrassed when King George VI died and nanny put black armbands on our coats as a sign of mourning. No one else did. And nanny said you have to because that is a mark of respect.”

As time passed, she became aware that her links to the great man could intimidate others. “I was rather a naughty child,” she recalled. “And when, as a schoolgirl, I was invited to my grandfather’s 80th birthday party, my boarding school headmistress told me, ‘If you do not behave yourself, I won’t let you go.’ And I remember a feeling of power, thinking, ‘You wouldn’t dare.'”

At age 13, Ms. Sandys was invited to join her grandparents, her uncle Randolph, and others on a cruise aboard Aristotle Onassis’s yacht, the Christina. On the trip from Monte Carlo to Istanbul, Ms. Sandys witnessed the shipping magnate openly wooing the opera diva Maria Callas in front of his wife Tina.

“Callas caused mayhem all the way,” Ms. Sandys said. “None of us liked her. I noticed that she did not wash her hair in three weeks. She behaved in such an insulting way, but it was fascinating to watch the romance unfolding before our eyes.”

Of Callas’s husband, Ms. Sandys recalled: “He was a womanizer who sued to wear white shoes. He played footie-footie with women under the dinner table. You could tell who he had been flirting with because the white used to come off on their shoes.”

And the trip generated plenty to talk about. “Onassis’s wife was delightful, but let’s not pretend it was an idyllic marriage. At the end of the day, my grandmother, my mother, and I would get together and discuss the gossip of the day. It was, for me, a defining moment. I had joined the grown-ups.”

Ms. Sandys began writing her series of books about her grandfather in 1993, after her uncle, Churchill’s nephew Henry Winston “Peregrine” Spencer-Churchill opened a battered tin trunk containing a large stash of forgotten letters, diaries, and documents. Most of them were from her grandfather. Her instant response was, “Someone ought to do something with them.” To which Peregrine responded, “Yes, why don’t you?”

So she decided instead to write from her unique perspective — a Churchill on Churchill. “I wanted to write something that hadn’t been written,” she said. “There was no point in competing with [Churchill’s official biographer] Martin Gilbert, who had written what I think of as the bible.”

She has now written three biographies. She also conducts tours of her grandfather’s favorite places in Morocco and South Africa, and hosts a Churchill-related cruise aboard Onassis’s yacht.

And though she shares her memories, the private time she spent with the great man will always be her own. When Harold Macmillan sent an RAF Comet to retrieve the ailing national hero from Monte Carlo, Ms. Sandys accompanied him on his last journey home. Some time later, he again fell ill and slipped into a coma.

“I remember holding his hand,” she recalled. “He lay there with his marmalade cat curled up beside him. He had insisted he should die in England and predicted he would die on the anniversary of his father’s death — and almost to the minute he did, on January 24, 1965.”