When Frida was going through pictures for Abba: The Official Photo Book, in 2014, she found an image of herself sitting in front of a mirror. ‘Agnetha and I did most of our make-up ourselves. It became a sort of meditation, a way of preparing to meet the audience. Each stroke of the make-up brush transformed me from private Frida to stage Frida. I was ready to step into the limelight and give it my all.’

Abba’s unique sound hinges in large part on Frida often being at the very limit of what her voice can handle and having to adopt a style of singing outside her natural range. But then, her entire life has been characterised by metamorphoses.

The Abba sound arises when Agnetha and Frida sing together, in the meeting of a soprano and a mezzo-soprano, a light voice and a somewhat darker one. Björn and Benny discovered that the unison singing was at its most powerful when the key was just a bit too high for Frida, so that she has to audibly fight to raise herself to Agnetha’s level. The fusion between Agnetha’s effortless and crystal-clear voice and Frida’s sweat, muscle and soul creates an indestructible alloy.

Anni-Frid Synni Lyngstad was born on 15 November 1945 in Ballangen, Norway, close to Narvik, above the Arctic Circle. An initial metamorphosis was necessary immediately after her birth. Anni-Frid was conceived towards the end of the Second World War, when Norway was occupied by Hitler and Narvik was one of the most important ports in Europe. Her mother was Synni Lyngstad, a nineteen-year-old Norwegian girl. Her father was the seven years older Alfred Haase, a sergeant in the German Nazi army.

Frida was a lovechild, but there is also a political dimension: during the war, German soldiers were encouraged to have children with Norwegian women. But by the time Frida was born, the war was over and so-called ‘German babies’ were targets for acts of retribution.

In 1935 the SS chief Heinrich Himmler had created the breeding programme Lebensborn, as a way of securing German world domination in the long term. In France, the number of children born as the result of relationships between French women and German soldiers is estimated to be 200,000.4 The French–Japanese cinematic masterpiece Hiroshima mon amour, written by author Marguerite Duras, tells one such story. However, Adolf Hitler disapproved of soldiers having relations with ‘women of a different race’ and thus discouraged this development in Latin countries like France.

Not so in the occupied Nordic nations, the population of which was considered perfect for ‘breeding’. In terms of administration, Norway was tied more closely to the Third Reich than Denmark, which was at the time still formally under self-rule.

Historian Folke Schimanski has written about Lebensborn in Norway. The idea that the programme would have consisted of actual stud farms is a myth disseminated by pulp novels with pornographic elements. Schimanski rejects this idea, saying that women were not actively recruited to be impregnated; the children were the result of romantic relationships.

The first Lebensborn home outside of Germany opened in Norway in August 1941. In total, nine such homes were established in Norway, four of which had maternity wards. The homes were not intended to bring soldiers and Norwegian women together, rather they existed to care for mothers who had become pregnant through romantic relations with Germans.

During the occupation of Norway, several hundred thousand German soldiers were stationed in the country – one German soldier for every eight Norwegian citizens. Most Norwegians despised the Germans and wanted nothing to do with them. Frida’s maternal grandparents, Simon and Anni, were active in local politics and leaned left politically; they were strong opponents of the Nazis, even though they had to keep their views secret. Simon passed away from cancer in February 1941 and Anni was left to support her children alone, among them her youngest daughter Synni, who would become Frida’s mother.

In Frida beyond Abba by Remko van Drongelen, the book that most elaborately describes Frida’s childhood and family history, Synni is described by her sister as calm, quiet, beautiful and known for her singing voice.

In the autumn of 1943, twenty-four-year-old Wehrmacht sergeant Alfred Haase arrived from Karlsruhe. He was tasked with organising the construction of defensive fortifications around the fjord and taking care of the German recruits arriving by train through Sweden. The way the war was going, a German victory no longer looked like a given. The German army suffered enormous losses in the Soviet Union, which shared a northern border with Norway.

After his first, long, Arctic winter, Alfred Haase spotted the teenage Synni Lyngstad in 1944. The German, who in photos has a moustache and wears his hair styled like Clark Gable, made sure to pass the Lyngstad house every morning. Synni could often be seen in the garden, where she sang as she tended to the fruit trees and flowers. Frida’s biographer Remko van Drongelen draws no parallels to The Sound of Music, but it is hard not to think of the scenes with the young man in uniform who falls in love with Liesl Von Trapp and then betrays her and her family to the Nazis.

Alfred began to court Synni. He neglected to mention that he already had a wife back home in Germany, Anna, and a newborn daughter, Karin. Synni spoke pretty good German. At the time, schoolchildren in the Nordic countries studied German as their second language, rather than English. When Synni told her sister that she was seeing a nice German man who played the accordion, she was immediately counselled to end the relationship. The rest of her family also tried to change her mind. But Synni was in love.

She and Alfred took long walks in the countryside and made plans for where they would go after the war, when they could travel again. Their flirtation grew into a full-fledged love affair. They stopped sneaking around and began to appear in public as a couple. People started to talk behind their backs.

In 1944 it finally happened: the Soviet army managed to cross the Norwegian border; the fortunes of war had changed. Alfred was immediately transferred to the naval base in Bogenviken, west of Narvik.

In January, Norwegian troops, flown in from Great Britain and Sweden, started to fight back against the Germans, who were now being attacked from two sides in the north. Alfred’s platoon was ordered to prepare for evacuation. On his last evening in Norway, he managed to bike all the way to the Lyngstad house and spent the night there for the first and last time. They got to say goodbye. At this point, Alfred had told Synni about his wife and child at home, but it had not stopped them from continuing their relationship.

At four in the morning, Alfred biked back to his platoon to be evacuated.

When Germany capitulated on 8 May 1945, Synni could no longer conceal her growing belly. Everyone in the area knew what had happened and who the father was. When Synni showed herself in town people hurled epithets and spat at her.

All around Norway, as in the rest of Europe, the celebration of peace soon turned into anger at what had taken place during the occupation. Women who had been in romantic relationships with German soldiers were labelled ‘horizontal collaborators’. Many were harassed, had their hair forcibly shaved and some had swastikas painted on to their faces. In Norway they were called tyskertøser. Pregnant Synni and her mother, Anni, were accused of being collaborators and forced to clean buildings in which the Germans had lived.

After Frida was born, Synni thought everything would work out. She waited hopefully for Alfred to return. But as she walked around with her stroller and experienced harassment directed at her child, who was called ‘German baby’ and ‘Nazi spawn’, grandma Anni realised that the situation was untenable.

They decided that Anni and Frida would go to neighbouring Sweden. Synni had found work at a hotel in southern Norway where nobody knew about her past. The plan was for Synni to make money and then join them in Sweden. By travelling alone, she managed to avoid having a ‘T’ for tyskertøs stamped in her identity papers. Many women in her situation were stripped of their citizenship and forced to move to Germany.

Getting to Sweden was complicated for Anni and Frida. The port in Narvik was blocked by capsized and destroyed vessels. After more than a week of travel by train and bus, grandma Anni and nine-month-old Frida arrived at a farm in the province of Jämtland in northern Sweden, where Anni was hired as a housekeeper for a widower.

Synni also managed to get to Sweden in the end and found work on a farm in neighbouring Härjedalen. Now both Frida’s grandmother and mother had an income and the three generations of Lyngstad women were close to being reunited. But it was...