Between 1934 and 1939, two "nervous British spinsters" were regular visitors to the opera houses of Germany and Austria. But the trips also served a greater and more dangerous purpose - saving Jewish lives.
Ida and Louise Cook risked their own lives dozens of times by smuggling out valuable goods for those attempting to flee the Nazi regime, as well as passing on messages and meeting contacts, some of whom were active in the underground movement.
Every time they left Germany, a border guard could have called into question the ownership of the furs and jewellery they were draped in, putting the sisters in peril of arrest and imprisonment - and worse.
They met one contact almost under the nose of several high-ranking Nazi officials, when he got in touch with them at their hotel - one of Hitler's lunchtime haunts - asking them to rush outside and jump into his taxi.
The sisters helped many people to escape, but as Ida said many years later: "The funny thing is we weren't the James Bond type - we were just respectable Civil Service typists."
The Cook sisters were born in the early 1900s, and - in common with a lot of their generation in the wake of the deaths of so many young men during World War One - never married, living quietly at home with their parents in south London.
Their passion in life was opera, and they became ardent fans - in the words of their nephew John Cook, "opera groupies" - attending performances and waiting at stage doors in the hope of getting autographs.
They also wrote to their favourite opera stars and in 1934 visited the Salzburg Festival, where they were befriended by the conductor and impresario Clemens Krauss.
It was his wife who suggested they "look after" a friend who was visiting London and she opened their eyes to the situation Jewish people living in Germany and Austria faced at the time.
Speaking in a BBC radio interview in 1967, Ida Cook said: "I can't emphasise sufficiently how we stumbled into this thing.
"This friend opened our eyes into the appalling situation Jewish people in Germany found themselves in. They were without any rights as human beings at all."
There were restrictions on all aspects of life and although Jews were at this stage able to leave the country, they were unable to take money or possessions with them.
Those wanting to come to Great Britain had to prove they had a job to go to or sufficient funds to live on - the latter presenting huge difficulties due to the money prohibition.
Ida and Louise were able to help them get around this Catch-22 situation by smuggling their valuable goods across the border.
The sisters would arrive in the country plainly dressed without coats, travelling home decked in jewellery and furs, which they pretended were their own.
They also persuaded people to vouch for refugees, by offering work or financial guarantees, to satisfy the British requirements for immigration.
Soon the Cook name went around and they began getting "the most pitiable appeals" from people wanting to leave.
At that point Ida - then earning £5 a week as a shorthand typist - wrote what she described as "a light romance" which was published by Mills and Boon.
Over the course of the next 50 years, under the pen name of Mary Burchell, she wrote about 130 novels for the publisher and "the money just kept coming in".
During the late 1930s this funded the sisters' trips to Germany and Austria.
When their frequent visits aroused suspicion, Krauss, who was then director of the Munich Opera House, stepped in.
He arranged for performances - sometimes letting them choose which one - in the cities and on the days they requested.
This allowed them to do their work amid what Ida later described as "some of the greatest operatic performances of the century".
They were careful to arrive and depart via separate border crossings, as it would have given them away if they arrived with very little luggage but left two days later "loaded to the Plimsoll Line" with jewellery.
Earrings had to be clip-on ones, as neither had pierced ears, as "that was the kind of thing officials noticed".
And they had their cover story ready.
Ida said: "Ours was very simple - we were two nervous British spinsters who didn't trust our families at home and so when we went abroad we took all our jewellery with us.
"There's no answer for that. You can say 'how ridiculous', but you can't say 'it's not true'."
And the danger was very real - they were breaking clearly defined Nazi law.
Sunderland historian Stuart Miller said: "They were smuggling large quantities of valuables knowingly and deliberately and could have been arrested at any point at the border, in which case there was nothing the British government could have done.
"There are examples of British citizens who were incarcerated and later ended up in concentration camps."
He came across the women's story when Ida's memoir was brought to his attention as he researched local heroes.
"They left Sunderland early, but they did live there - and it was where Ida was born," he said.
"Children in schools can identify with their story; not only were they local but they were two young women, doing this in a period when women were just coming out of the Victorian and Edwardian age.
"It's almost a Scarlet Pimpernel story."
As a permanent reminder of the sisters' heroism, on Friday Sunderland Council unveiled a blue plaque at the entrance of Croft Avenue, which was their childhood home.
Joining local dignitaries for the ceremony, Tony Wortman, chairman of the Newcastle Reform Synagogue, said: "Ida and Louise Cook saved many Jews... many of whom are still living and now surrounded by their children and grandchildren.
"For them, life has gone on but this would have not been so had it not been for their bravery and I am pleased to be invited to honour them on behalf of all those they saved."
In 1965, the sisters received the Righteous Among the Nations honour from the state of Israel, for their "warmth of heart, devotion, rare perseverance [and willingness] to sacrifice their personal safety, time and energy".
Ida died in 1986, with Louise outliving her by just over four years.
Remembering the "quite extraordinary old biddies", nephew John Cook said: "They were modest, only mentioned their exploits if asked.
"They were first really exposed when Ida's wrote her memoirs, which was mostly about operas.
"The story of the rescues was in the context of opera visits; there was no desire to push them to the forefront."
He added: "Ida used to say something like: 'You are what you do' and that was about it.
"They did it because it was the right thing to do, nothing more, nothing less."