As a young girl, Ariana Neumann wanted to be a detective. She loved solving puzzles and mysteries and even ran a spy club with her cousins and some friends in the garden of her Caracas home. But it was not until her Czech-born father, Hans, was seen awkwardly moving a box that anything significant emerged from their spying operations.

Its contents, which included a pink identity card with a Hitler stamp, issued in Berlin in the name of Jan Šebesta but with a photograph of her father as a young man, would, decades later, take Neumann on an extraordinary, poignant, personal journey of discovery, which she chronicles in her widely acclaimed, captivating and elegantly written memoir, When Time Stopped. Researched with dogged tenacity, Neumann unravels the secrets and truth about her father’s past and wartime survival: from his escape in 1943 from Nazi occupied Prague and his hiding in plain sight under an alias, working for a Nazi-owned paint factory in war-torn Berlin, to the lives and fates of relatives that she had never known about.

Hans Neumann was a hugely successful Venezuelan entrepreneur, philanthropist and collector of watches and art yet, in many ways, was also an enigma. He never spoke about his past, not even that he was Jewish — Neumann was brought up nominally Catholic. At 17, a chance comment by a fellow undergraduate alluding to her Jewishness because of her surname totally shocked her. She had never considered her religious identity until then. When she told her father, it was made clear that the matter was not for discussion.

But when he died in 2001, he left Neumann the box of documents and photographs that she had first seen as a child. It was deliberate, she tells me, when we meet at the Kensington home she shares with her husband and three teenage children. Her father’s office, which had always been full of papers, from phone messages, notes and receipts to files with people’s names, had been completely cleared out. “Visually, it was an astounding moment because I’d been there five months prior and it was crammed. I was ready to spend two weeks full-time sorting through his papers and they were all gone, except for this box.”

She believes that he was giving her the permission to investigate his earlier life and write what he himself could not bear to reveal. There was no risk that she would not find everything out, she says. “He knew me very well. I think he knew that I was going to do it.”

Although Neumann describes having a deep relationship with her father, she says he did not really speak about feelings, not even about the death of her brother, Michael.

“He kept everybody a little at bay, so his relationship with me was about doing things together, finding things that we both liked and enjoyed, such as puzzles and ideas, and we would bond through those moments.” But he was very regimented, she says, and by the time Neumann was a teenager, if she wanted to speak to him, she had to make an appointment.

She now realises that his behaviour was a protective mechanism, that he was scared she might ask questions he did not want to answer.

“He lost a lot of people that he loved,” she explains [25 members of his family were murdered in the Holocaust] “and I think the protective wall was because he thought that, somehow, he could manage his feelings better. And, if he didn’t ask questions about my life, then I wouldn’t ask questions about his.”

Over the years, Neumann unearthed a myriad of documents, photographs and diaries, including letters written by her grandparents to her father and his brother from Theresienstadt — they were eventually killed in Auschwitz. There were moments when reading them was too upsetting for her and so she would pull back. “I was always curious, but I didn’t feel bold enough. I was scared of the darkness in the letters and the story and I wasn’t sure what I was going to find.”

However, as time passed, “I found that if I just read on a little more, I got a little bit of their essence and I liked that. It allowed me to have these grandparents that I knew nothing about. I got an intimate glimpse into their marriage, the people they were, and I fell in love with them both.”

The physical experience of retracing her family’s footsteps has been both joyous and traumatic. Visiting Prague as well as her grandparents’ country house in Libice, a Prague suburb, was filled with beautiful moments, she says, smiling. “Prague is remarkably the same and I also know the family were very happy in Libice.”

Walking around Theresienstadt was obviously much more difficult, but, “yet again, I could feel my grandparents there. Maybe because I’d just spent so long reading the letters and because they actually, in all the horror, retained their humanity and there were little titbits that allowed me to piece them together.”

The train journey from Prague to Berlin— the same one that her father had taken in 1943— proved difficult. She had not realised that the train weaved so close to the house in Libice and hoped he would not have seen it, as he had travelled in the middle of the night. “I cannot imagine how terrified he was and how broken hearted he must have been,” she says. Once she arrived in Berlin, she was desperate to leave. “Despite knowing the city welcomes immigrants and it’s inclusive, vibrant and cultured, for about 20 minutes I couldn’t get 1943 out of my head. It was just awful. I thought I wouldn’t be able to stay.”

The process of uncovering her paternal family’s history, has changed her. “It has made me so much more aware of antisemitism and the insidiousness of racism in our everyday lives and language,” she says. “I do feel that having done what I’ve done, I need to fight it, even if it’s Sisyphean, to make people aware.” She feels strongly connected to Judaism. “It resonates with me in a way that no other religion does,” she says, her voice briefly breaking with emotion. “If I was going to be religious, I wouldn’t hesitate to convert.”

Among the innumerable papers and photographs that Neumann found were also some objects including a ring, fashioned out of copper pipe, made in Theresienstadt by her grandfather for his daughter-in-law, Zdenka — her uncle’s non-Jewish first wife — as a thank you for her bravery and kindness.

She sent parcels to her in-laws and even smuggled herself into the camp on two occasions, making direct contact and carrying essentials for them. The ring has become Neumann’s talisman. “I wear it for luck, for grounding and direction. It makes me feel connected to them, reminds me to keep things in perspective and to really find the beauty in everything because it’s there.”

‘When Time Stopped’ is published by Simon and Schuster UK