Together, the pair have done so much good in their relentless pursuit of Nazis around the globe and efforts to preserve the memory of millions of Jewish lives.

In their memoir, Hunting the Truth, translated from French into English by Sam Taylor, we hear directly from them in alternating chapters that reveal them to be both humble and private.

They seem irresistibly drawn to each other and their life’s work with an almost symbiotic sense of mission that has kept them unusually close.

Their eldest son, Arno, lives above their apartment in Paris and checks in on them frequently, as Beate and Serge are now 79 and 82, respectively.

Their daughter Lida and grandchildren live close by.

They have focused their activities over the past 55 years on bringing to justice the Nazis responsible for sending 75,000 French Jews — including 11,000 children — to their deaths.

Serge would have been among them if his father had not had the foresight to build a hiding space behind a kitchen cupboard, where Serge hid with his mother and sister while the Gestapo took his father away.

His father was later killed in Auschwitz, which pains Serge to this day.

After the war, Serge met Beate — who grew up in the ruins of a defeated Nazi Germany — on a Paris metro, and they fell madly in love.

Their life mission was cemented for each of them when Beate had the chutzpa in 1968 to slap the face of the West German chancellor Kurt Georg Kiesinger, while calling him a Nazi in the middle of the Christian Democratic Union party conference.

Later that year, Beate chained herself to a bench in Bolivia alongside a woman who had lost her three children to the Nazis, to protest Bolivia’s sheltering of Klaus Barbie.

Beate explains her childhood evolution as the daughter of Nazi sympathisers with her customary bluntness: “We avoided speaking about Hitler.

“Prior to 1945, I used to recite little poems for the Führer at my kindergarten. I lived in the ruins, but I didn’t know why Berlin had been destroyed . . .

“The world where I grew up was never explained to me.”

In 1960, at age 21, she left Berlin for Paris to work as a nanny.

She said she remained haunted by feelings of guilt and shame about the behaviour of her parents and fellow Germans.

She explains the complex set of emotions that prompted her to become an engaged and passionate activist for righteousness.

“The role I play is much bigger than I am,” she said. “Inside me, there is the black of a Barbie and a Kiesinger: there is the gray of those who, out of indifference or cowardice, resign themselves to the impunity of Nazi war criminals or the repression in Prague; and there is the off-white of those who, though they are not resigned to such horrors, are content to sign petitions in order to appease their troubled consciences.

“And yet what counts are acts — black or white — and the choice of principles that lead one inexorably to act in a way that is black or white. Each man’s fate is determined by his acts.”

Beate also credits her husband for helping her develop into someone more learned and contemplative.

She reveals that their early love letters were filled with his encouragement to enrich her mind and immerse herself in the works of Dostoyevsky, Tolstoy and Stendhal, writers he revered and believed wrote to enhance the awareness of others.

Serge is still haunted by the last day he saw his father alive. The family was living in Nice in 1943 in the Italian zone and believed they were temporarily safe.

But those feelings of security were shattered when his father was taken away by the Gestapo and murdered at the age of 39.

“That night has stayed with me all my life — as it has for all Jewish children who experienced terror and lost loved ones — as an experience that forged my identity as a Jew,” he writes.

“I did not inherit that identity through religion or culture: my Jewish identity is forged by the Holocaust and an unswerving attachment to the Jewish state of Israel . . .

“I have a superficial knowledge of Jewish history, but I do not speak Yiddish or Hebrew. And I am not a believer. And yet, I am a Jew.”

Serge believes his father must have been horrified by the identification number that was tattooed on his arm before his death, erasing his identity and then allowing him to vanish into scattered ashes.

The book chronicles the Klarsfelds’ greatest feats.

The couple took dangerous risks and once had their car blown up by a suspected neo-Nazi group.

Beate was often away from her two children and overwhelmed at times with worry for her family’s safety. So was Serge.

Each of them faced several arrests; time in prison; and rough interrogations in countries where the rule of law is still a pipe dream. But they could not and would not stop. Their son, Arno, soon joined them in their efforts.

They were responsible for the arrest and life sentencing of Klaus Barbie, who was found guilty of crimes against humanity.

They chased Barbie for over a decade and finally had him brought to France in 1983, after he had been hiding in South America for 32 years.

Barbie’s crimes included the seizure of the children of Izieu, the deportation of at least 78 Jews arrested in a Gestapo raid, and the organisation of a train convoy carrying 650 children, that included Jews and members of the Resistance, to concentration camps in Poland and Germany.

The Klarsfelds were also responsible for the conviction of Maurice Papon, who helped send more than 1,600 Jews to die in concentration camps; and the life sentence of Paul Touvier, an aide to Barbie accused of executing seven Jews and crimes against humanity.

They also led some more unsuccessful attempts to bring Nazis to justice, including Alois Brunner, who lived in safety in Damascus after the war and is believed to have died there.

They also failed to capture Kurt Lischka, a West German judge who had once run the Gestapo in Paris and was responsible for the deportation of 75,000 Jews.

He was later convicted and jailed in Germany but was released early for health reasons before dying in a nursing home in 1989.

Both Serge and Beate worked tirelessly with the French and German governments to change the existing laws, so that Germany would be permitted to extradite former Nazis and try them in Germany for offences committed outside Germany.

Serge’s work as a historian and documentarian uncovered invaluable documents that revealed that Philippe Pétain was directly behind the anti-Jewish measures when he willingly aligned himself and France with the Nazis’ racial ideology.

This ended years of debate between historians who grappled over whether Pétain’s Vichy government was a puppet of the Nazis or an eager and active participant, collaborating with the Nazis.

The couple were moved by French president Jacques Chirac’s 1995 proclamation to Jews, an apology that was long overdue.

And in 2012, they rejoiced when president François Hollande said at the commemoration at the Vel d’Hiv, where Jews were rounded up and shipped to Auschwitz: “The truth is this crime was committed in France, by France.

“More than 80 per cent of the Jews deported from France were arrested by the French police.”

Serge is most proud of his masterwork, Le Mémorial de la Déportation des Juifs de France, a giant telephone-book-size directory that simply lists the names and addresses of all 75,000 deported Jews.

He later wrote a much revered book, French Children of the Holocaust, which is almost 2,000 pages long and tells the story of each French child who was killed.

When asked to explain why he took on such a project, he said: “I wrote it maybe because I was a survivor.

“I was able to escape death, but I didn’t escape it for nothing. I didn’t want what happened to others, and what could have easily happened to me, not to be known.”