Nicholas Shakespeare was always intrigued by his aunt Priscilla, who had a glamorous if mysterious presence in his childhood. After her death, he delved into her past in wartime France and found she had been hiding a dark secret

My aunt Priscilla, my mother's sister, was a figure of unusual glamour and mystery in my childhood. She lived on a farm on the West Sussex coast with her second husband, Raymond, a mushroom farmer and a jealous man who never let her far from his sight.

Priscilla invited us for weekends at their home in East Wittering and whenever her name was mentioned on the journey from London I craned forward in the back of my parents' car. From an early age, I was conscious that my aunt was the sort of woman that men fell for. Both my parents loved her, but were unable to puzzle out the riddle of her relationship with Raymond.

Inevitably, as our mauve Singer Gazelle turned into the lane leading to Church Farm, there would be speculation about how late we were going to be and whether Raymond, a tyrant for punctuality, would – this time – serve mushrooms. The promise of a mushroom is hard to recapture today; cheap production has rendered the taste mundane. But to a seven-year-old boy in the early 1960s a mushroom was a fantastic thing – almost as exotic, in its way, as my aunt.

Priscilla's home was a redbrick farmhouse built next to a 12th-century church. There was a courtyard with stacks of empty fish boxes for growing the mushrooms in. The "growing rooms" were sinister-looking Nissen huts, 30 of them side by side. I was under firm orders not to enter. My shoes risked picking up a dangerous virus called La France disease, which, if spread, could wipe out Raymond's crop. So I never saw inside a hut. But I remember buckets of disinfectant and the damp, musty smell of compost. "Kept in the dark and fed on shit," was Raymond's formula for a successful flush.

The extent to which Raymond controlled Priscilla was blatant even to me. When you enter a room and everyone's talking, you end up being drawn to the silent one. Even though I was only a child and Priscilla a woman in her late 40s, I felt protective of her.

Priscilla spent long periods alone in her room upstairs. I have a vivid memory of the room because at the foot of the double bed was the first television I laid eyes on.

From the beginning, I am sure of two things. First, her sheer attractiveness. She reminded me of Grace Kelly in one of the films I watched in her bedroom. She laughed, and I remembered my grandfather, his smoky laughter rising across the South Downs. Her laugh was rejuvenating and I noticed that my parents changed in her company, perhaps returned to the young man and woman they were before they had children, when they lived in France. In a strange way, she was the delicacy that we went to Church Farm always hoping to savour, our champignon de Paris.

The second thing I am sure of was her sadness. She seemed weighed down by a past that I could never work out, and nor could my parents. My father said: "I suspected she'd had an extraordinary past, but she never spoke about it and one would never ask her." This aloof, indefinable sadness was her bedrock.

My parents gave me some basic facts. Priscilla had grown up in Paris, where she had trained as a ballerina. She had worked in prewar Paris as a model.

She had lived in France during the occupation and spent time in a concentration camp. My mother said: "That's what I was told by her when I was 17 – at Church Farm. She was captured and tortured by the Germans. I presumed she couldn't have children because she had been raped and caught an infection."

She had been a vicomtesse; her first husband, an aristocratic Frenchman who never ceased to love her. I was curious about his nickname for her – "my little cork". Why he called her this was not explained.

Nicholas Shakespeare. Photograph: Ekko Von Schwichow

Priscilla died in 1982, but her fate obscurely moved me. What had she done in the war? Why did she not return to England after getting out of the concentration camp? Why did her father, SPB Mais – a well-known author and broadcaster – never mention on the airwaves or to my mother the fact that his eldest daughter was isolated throughout the war in occupied France? I pictured her crouched before an illegal radio-set in a Paris atelier, listening to my grandfather's voice on the BBC. A pioneer of radio, he had inaugurated the Kitchen Front series, and in 1940 received up to 500 fan letters a day, causing even Winston Churchill to say of him: "That man Mais makes me feel tired." Did he ever transmit to Priscilla a personal message that only she could interpret, like one of those mystifying coded messages to the resistance, such as "The hippo is not carnivorous"? Could she have been in the resistance?

A year or so after Priscilla died, my mother revealed to me that she had been kept in the dark about Priscilla's existence until the last months of the war. She was in her first term at Cheltenham Ladies' College when she received a letter from her father, who, pricked into action by Priscilla's return from occupied France, was writing to break the news that my mother had an older half-sister and that her father had been married before. The reason he'd had to keep this secret, apparently, was because the BBC refused to employ divorced people; he was afraid for his job.

About the upbringing of her sister, and about the circumstances of Priscilla's life until 1944, my mother admitted that she possessed, even now, only the haziest outline. The idea that my mother had been unaware of her father's other family until the age of 13 was too irresistible not to follow up and I contacted Priscilla's stepdaughter, Tracey. I explained how curious I still felt about Priscilla and asked if my aunt might have left behind any personal papers. "It's odd that you should ring up now," Tracey said. She had in her possession a cardboard box filled with photographs, love letters, diaries and manuscripts, including a stab at a novel, which Tracey had salvaged soon after Priscilla's death, from the striped padded chest at the end of her bed – on which the telly used to sit. The papers all related to Priscilla's time in occupied France. They were more or less everything that someone seeking to unravel Priscilla's enigma could hope for.

The story that unfolded from this box of papers was one of an ordinary woman in extraordinary circumstances. I discovered that Priscilla, after leaving the internment camp at Besançon, had gone underground, but not to join the resistance; rather, to dissolve into the crowd. And if, to keep warm and secure a meal, she did things that Priscilla Mais or Vicomtesse Doynel de la Sausserie might have known in her inward conscience was wrong, then she was also part of the mass.

This is not a criticism or disparagement. The impulse to cast people as heroes or traitors ignores the muddled and shifting reality of the overwhelming part of the population who drifted with the stream. Prudent, unaffiliated, not committing themselves to resistance or collaboration, not fitting into a neat moral category, playing a number of ambiguous and provisional roles, ready at any instant to change direction with the current. At this dark moment in France's history, a friend of André Gide said he felt "like a cork floating on the filthiest water". That summed up Priscilla.

She was one of remarkably few English women to have lived in Paris through the occupation – perhaps one of fewer than 200. She learned what it was to be faced with decisions that her family and friends in England never had to confront, yet which they judged others for having made.

Her story turned out to be not about an elite coming to terms with fascism, but about ordinary people – ordinary women especially – adjusting, screwing up and developing survival skills of a deeply primitive and totally understandable, if ruthless, kind. This included sleeping with the enemy.

One evening, after piecing together her sister's story, I telephoned my mother. I told her that it looked as though one of Priscilla's lovers in Paris, who had invited her to dine at Maxim's and dressed her in Schiaparelli and Patou, may have been the prominent Nazi intelligence officer in charge of Bureau Otto and had overseen the systematic plunder of France. My mother's reaction? "Nothing would surprise me in the war. Absolutely nothing. It's a question of survival. You never knew who you were going to meet, and you lived from day to day. I'm sure that you would have collaborated if you had wanted to live."