Documentary filmmaker Doug Block found this love letter tucked into the guidebook he’d taken along on an anniversary trip to Paris. (Doug Block)

Henri loved Betty, and Henri loved Monet. The rest is a mystery.

Like all good mysteries, this one features an elusive woman, a love affair and a random stranger who gets swept up in the story. But there’s no ending — yet.

In November 2015, Doug Block was packing for a 30th-
anniversary trip to Paris with his wife, Marjorie Silver. The New York filmmaker pulled a used tourist guide from his bookshelf and threw it in his luggage. On the last day of their trip, he grabbed the old copy of "Pariswalks," and out dropped a cream-colored envelope with "BETTY" slashed across the front.

He turned over the envelope and took out a card. The cover featured an impressionist painting of a woman sitting on the grass. He flipped the card open.

My Sweet Love, began the inscription. Will you look for me at the Musee D'Orsay ? I will be there in soul and spirit, though not in body. It is there that you will find my love of Monet. It is there, my love, that I hope you find me and find such peace and beauty in a city where I would like to be — with you.

After the Mussee D'Orsay, go have a café au lait at the street café to the left.

I love you more than words can tell, my sweet angel. You are the source of my greatest joy.

Not going to Paris with you is so hard. It is meant for two. It is a city of love. Please keep me close to your heart, every step of the way.

Enjoy, my love.

Avec Amor , Henri.

Who was Betty? Who was Henri? Block turned to his wife. “Where did we get this book?” he asked.

And so an obsession was born.

Doug Block filming the Monet painting “Woman Seated under the Willows” at the National Gallery of Art. (Doug Block)

Block, 64, is an award-winning independent documentary filmmaker who makes features about family, marriage and other human foibles. He was in the middle of a project about his fellow documentary filmmakers, but the love letter haunted him.

And so he decided to make a film about Henri and Betty. Or, to be exact, his search for Henri and Betty.

“It gave me a chance to play detective and solve a mystery that I felt perhaps is unsolvable,” says Block. “But I sensed that playing detective, it would lead me down really interesting, intriguing paths.”

He began with the guidebook, thinking that it must have originally belonged to Henri or Betty. It was published in 1999 and had been sitting on his bookshelf for years. None of his friends could remember lending it to him. Maybe he’d picked it up at a used bookstore? The only thing he could know for sure was that it was purchased in 1999 or later.

He turned to the letter.

Block dismissed the idea that Betty was a relative or a friend — the words are too intimate, dripping with longing. He thinks that the two were lovers, and the guidebook was a gift to her, the card tucked inside as a surprise. This may have been Betty’s first trip to Paris, with Henri directing her to visit the Musée d’Orsay and go to a favorite cafe.

The filmmaker also believes that “Henri” was probably a lovestruck affectation of Henry, and that Henry was an American. Block points to the misspellings of “musée” in the second reference and “amour” in the closing — mistakes that a native French speaker would not have made. Based on his poetic language and striking penmanship, Henri was sophisticated, well-traveled and familiar with Paris.

A handwriting expert looked at the note and noticed the distinctive star-shaped “I” (which looks more like an “A”) and concluded that the author was brilliant but very careful about what he said and did. Betty was someone who allowed Henri to release a heretofore unexpressed passion. “It was like he opened his heart and this flowed out,” the expert told Block.

Betty, on the other hand, is a total enigma. No clues to her age, her profession, her given name. Elizabeth? Roberta? Since “Betty” has fallen out of favor as a girl’s nickname, she was probably born no later than the 1960s.

Block consulted a detective, who said that too many people had handled the card and envelope (which was never sealed) to obtain usable fingerprints. The detective recommended a psychic who had assisted him on a few cases.

The psychic held the card and started tearing up before he even opened it. “ ‘This is really poignant,’ ” Block says the psychic told him. “ ‘They really loved each other so much.’ He thinks this letter was early on in their relationship and they so wanted to be together. But they couldn’t and had to go back to their marriages.”

An archival researcher scanned Henri’s handwriting and looked for matches on the Internet, to no avail.

A friend was convinced that Henri was a serious Grateful Dead fan. "I love you more than words can tell," he writes — a common enough phrase, but more often rendered as "I love you more than words can say."

That last word makes all the difference: The former is a line from the Dead’s “Brokedown Palace,” a lover’s farewell. Block’s friend looked at the willows in the picture (historically a symbol of mourning) and said: “Oh my God! He’s not only a Deadhead, he’s either sick or dying. That’s why he couldn’t go to Paris with her.”

Block thinks that this letter is a farewell, too, but maybe just the end of the romance. There’s no reference to the future or of being together again. “To me, this seems like they have already called it quits,” he says.

Then he showed the card to a forensic analyst, who said that there’s no actual proof of anything. Henri and Betty could be lovers, or they could be husband and wife. Maybe Henri and Betty aren’t even their real names.

Everybody has an opinion, and no one has any facts. “But the whole fun of this is speculation,” says Block.

The front of Henri’s card is printed with an image of Monet’s painting. (Doug Block)

There was one clue left: Henri selected a blank ­notecard with a Monet painting, "Woman Seated Under the Willows," on the cover. The back of the card identifies the painting as property of the National Gallery of Art, which is one reason Block was in Washington recently. Maybe if he knew more about the art, he could know more about Henri.

The curators allowed Block to film the painting, one of more than two dozen Monets in the gallery’s collection. It’s a beautiful landscape — shimmering, delicate, the colors more vibrant than any image on a card can capture — with a woman reading on the banks of the Seine.

“There’s a wonderful softness and curvaceous quality,” says Kimberly Jones, curator of 19th-century French paintings. “Your eye kind of swoops and swirls as you go around the canvas.”

There’s a love story behind the painting, too. In 1877, Monet invited his longtime patron, Ernest Hoschedé, a department store magnate who went bankrupt, to live with him and his family in Vetheuil, 40 miles north of Paris. Hoschedé brought his wife, Alice, and their six children. It was all very cozy. Perhaps too cozy.

Monet’s wife died in 1879, and Hoschedé moved back to Paris to rebuild his business, but Alice and the children stayed with Monet. Monet and Alice eventually married, and experts think that Alice was the model for the woman under the willows, painted in 1880.

Originally, Block thought that Henri lived or worked in Washington, because the painting is part of the Chester Dale Collection and cannot travel or be exhibited outside the museum. He also guessed that Henri bought the notecard, printed in 1996, in the gallery gift shop.

But the dates don’t add up. The painting has not been on public view since 1993, and odds are that Henri didn’t buy the card at the museum at all.

Issued from 1996-2003, the image was part of a boxed set of 20 cards by Galison in New York, featuring four of Monet’s works from the National Gallery. According to Galison Chief Financial Officer Sam Minnitti, the firm sold more than 160,000 boxes in museums, bookstores, gift and stationery shops all over the country. Henri could have purchased the notecards anywhere.

And despite his professed love for Monet, maybe this particular card held no hidden meaning. Maybe he just liked the way it looked.

“I think it says he’s romantic,” Jones told Block. “There’s a wonderful frisson of possibility and potential, which I think he may have been responding to. There’s this woman in this beautiful setting, and she’s waiting. She’s waiting for him.”

The Musée d’Orsay in Paris, where Henri told Betty that she would find his love of Monet. (Ron Heflin/Associated Press)

Block didn't know any of this, of course, during his anniversary trip to Paris. On Nov. 13, 2015, he and his wife spent the day at the Musée d'Orsay. A few hours later, terrorists attacked and killed 130 people at a concert, restaurants and cafes, and a soccer stadium.

On their last day, the couple decided to go for a long walk through the now-hushed streets of Paris. Block pulled out the guidebook, and out fell the envelope.

“Paris was in the same state New York was in after 9/11: The world will never be the same again,” remembers Block. “It was always the city of love, now it was going to be associated with terrorism.” And so, he decided he needed to make a film about love instead of hate.

Maybe Henri and Betty are still alive. Maybe someone will see the love letter and recognize Henri’s handwriting. Maybe someone knows the end of the story.

But if he never solves the puzzle, Block’s okay with that, too. His film is really about love — love found and love lost. It’s about why couples stay together, and why they part. It’s about the quest, not the destination.

“I’ve come to realize I’m not creating a literal depiction of Henri and Betty,” he says. “I’m creating an impressionistic portrait. And everybody I’m talking to is creating the brushstrokes.”