(CNN) — A lost love letter, a famous painting and a trail leading through the streets of Paris -- do you have the answer to a romantic mystery that's been perplexing a US filmmaker for two years?
Doug Block stumbled on the strange case of Betty and Henri while he and his wife were celebrating their wedding anniversary in Europe in late 2015.
"It all started in Paris," the documentary maker recalls.
On the last day of their trip, the couple decided to go on a long stroll, savoring the fading autumnal daylight. Block fished out "Paris Walks," the tattered, second-hand guidebook he'd stuffed in his suitcase at the last minute.
As he leafed through the pages, an envelope fell out, emblazoned with a single name: "Betty."
"It just fluttered to the floor, almost like in slow motion," Block tells CNN Travel.
The envelope was unsealed. Inside was a note card, adorned with a Monet painting: "Woman Seated under the Willows." Written inside was a passionate declaration of affection.
"Will you look for me at the Musee D'Orsay?" wrote Henri, the mysterious author. "I will be there in soul and spirit, though not in body. It is there that you will find my love of Monet."
Block turned to his wife in confusion. Who was Betty? Who was Henri? And how did this letter end up in his guidebook?
Little did he know the letter would mark the beginning of an globe-spanning quest, an international labor of love celebrating Paris and the enduring power of romance.
A serendipitous discovery
Doug Block found a mysterious love letter referencing the Musée d'Orsay.
Thierry Chesnot/Getty Images Europe/Getty Images
Block and his wife traveled to Paris in November 2015. They spent their days wandering the wide boulevards, strolling the Seine and admiring the Impressionist greats in the Musée d'Orsay.
On their second evening in Paris, a Friday on November 13, the city was rocked by a series of deadly attacks in which 130 people died.
"I remember I had been thinking, 'Oh my God, Paris, the city of love and romance is now going to be associated with terrorism,' " says Block.
Unearthing the love letter seemed like a sign from fate -- a serendipitous reminder of romance in a moment of heartbreak.
"The love letter hit home," he says. "The timing of it, in terms of everything that Paris was going through and the whole response internationally to the terrorist attack."
Beginning the quest
The card is emblazoned with a Monet painting.
Courtesy Doug Block
Back home in New York, Block says he was "haunted" by the specters of Betty and Henri. However it wasn't until a year later that he decided to pursue these ghosts, feeling disheartened in a post-election, divided America:
"I just thought, 'for the sake of my sanity, I think I need to do something which is driven by love, and romance.' "
But how do you begin searching for two people, knowing only their first names and handwriting? Block's first step was to work out whether the guidebook had been borrowed from friends.
Block made this unexpected discovery on holiday in Paris.
LUDOVIC MARIN/AFP/AFP/Getty Images
This line of inquiry lead him to the door of a mutual friend in New York, a woman named Betty.
"It was not Betty, but it was a remarkable Betty," says Block. "She was 91 years old. She spoke very candidly about her love life [...] I just felt like, I think I'm going to find a whole succession of Bettys and Henris in this search."
Block's conversations with this Betty consolidated his conviction that this was a story worth pursuing. After all, the letter, with its heartfelt sentiments and evocation of lost love, resonates with everybody.
Plus, he adds "people just love a good mystery."
Block has analyzed the handwriting in the letter.
Courtesy Doug Block
Concluding that he and his wife must have bought the book secondhand, Block decided to further his investigation.
"I literally played detective by going to see a detective, that was really my first step," he says.
Since then, his quest to track down Betty and Henri has spread far and wide.
The Monet painting printed on the note card hangs in Washington's National Gallery of Art. Block has filmed the gallery's collection and explored how Monet's tumultuous love life could be linked to Betty and Henri's real life romance.
"It's gone to a forensic analysis and a handwriting expert, a historical novelist who wrote a book about Monet."
Block even employed a psychic. The filmmaker has interacted with a host of interesting characters, all committed to solving the puzzle.
A year on and Betty and Henri haven't been found -- but the search has taken on a life of its own.
"It became clear to me that it's a real long shot finding Henri and Betty, but the search is really the heart of the film -- and the fun of the film," he says.
Reality and fiction
Block had permission to film in Washington's National Gallery of Art.
Courtesy Salam Shahin
Block is still hopeful that Betty and Henri might resurface -- he encourages anyone who recognizes the handwriting to reach out to him on Twitter.
Nevertheless, he's aware the appeal of the couple is partly the romance, the fairy tale, the fiction.
Finding the reality -- which might be bleaker than it seems -- could be disappointing.
"There's something about the idealization of who they are," reflects Block. "What's so fascinating is that everybody I've spoken to has their own concept of who Betty and Henri are. When I ask them 'What's their age and what's their relationship?' It's all over the place."
For Block, who is in his 60s, Betty is Audrey Hepburn in her Paris movies -- an ingénue in an old Hollywood love story.
"We all carry these ideas of love and romance, often fed by movies [...] I think to find out they were just normal people, might be disappointing," he concedes.
"This may well have been an affair where they went back to their marriages. And that's tricky, because I don't want to necessarily expose them publicly."
Block says he would only publicly name the lovers if they gave permission, admitting it's a fine line to tow.
Block still hopes he might find the Betty and Henri.
LUDOVIC MARIN/AFP/AFP/Getty Images
In the meantime, Block will continue his search. He loves connecting the dots -- and connecting with others as swept up in the romance as he is.
"The story just seems to connect with everyone, this idea of the kind of message in a bottle aspect of it, of this note somehow getting into this book, somehow ending up in my hands. The beautiful sentiment that's expressed in the letter. The mystery of the note card and whether there's a connection -- Monet is somehow involved."
At its heart is a love letter with a universal message:
"Betty and Henri are pretty much everyone," says Block. "Everyone no matter what their age has this yearning for romance, even if they've been married for many years, it doesn't extinguish this flame [...] I think this letter taps into it for most anybody I've come across."