Harriet McKechnie and Tom O’Mahony were married June 28 on the beach in France where they met as teenagers more than 15 years ago. (Gilles Perbal)

It all started with an airborne bottle cap.

In the summer of 1999, Harriet McKechnie and Tom O’Mahony were high school students at a French language program in the south of France. Five days into the program, the American girl and the Irish boy found themselves on the beach with separate groups of friends. Unlike the girls, Tom and his fellow underage Irishmen had brought a few bottles of Desperados with them.

They were understandably concerned about being caught, but one of Tom’s friends forgot to keep an eye on the bottle cap he was trying to unfasten with a lighter.

Then, all of a sudden, it flew across the beach and hit Harriet on the head. She stomped over to his group with her arms crossed and demanded to know who had done the deed.

“It was then that I thought, ‘Wow, well look at this blonde-haired, blue-eyed American girl talking to us!’ ” Tom recalls with a laugh. “I thought it was very important to have this girl keep on talking to us but have her not mad at us.”

To make up for their foolishness, they invited Harriet, a Bethesda native, and the rest of her American friends over to sample the adult refreshments they had acquired. She remembers laughing until her “stomach felt like it was going to bust open.”

Over the next few weeks, Harriet and Tom grew to be friends and agreed to keep in touch after the program was over. Once Tom was back in Dublin, he sent her a letter. Upon receiving it, she sent one back, and they officially considered themselves pen pals.

For the next three years, Tom wrote notes to her in alternating shades of gold and purple sparkly gel pen — a detail Harriet refuses to forget — about what he was doing in school, his struggles and his dreams. Harriet, who was 17 when the letters started, remembers running to the mailbox every day to check if another had come so she could respond with her own high-school-age anxieties.

The pair exchanged five or six letters a year, with special cards around the holidays, until Harriet moved to England for college. Now they could call each other instead. Tom visited Harriet in London and she and her sister visited him in Dublin. Still, their correspondence stayed platonic, with the goal of making the other laugh, not swoon.

“Whenever I had a problem or any good news, I’d call him,” she says. “He was my best friend. You know, boyfriends would come and go but I knew he’d always be there for me.”

After graduation, Harriet returned to the States and found out he’d be spending his 24th birthday in Florida getting his pilot’s license. She decided to meet him there.

It was good timing because Harriet had grown bored of her desk job in Washington and was striving for a new career that would take her to places more exotic than the communal water fountain.

By the next year, she was a flight attendant for United Airlines. It was the career of her dreams, albeit with the downside of being in a constant state of jet lag, something she had to take a two-year break to get over. But the upside was travel and also being able to see Tom more. She went back and forth between Washington and London for three years. Then came their first opportunity to turn their relationship into something more serious.

In the summer of 2011, 12 years after they met, Harriet made London her permanent home. To celebrate, Tom took her to a swanky club where, upon entering, she fell down an entire flight of stairs. In need of a place to recover, they headed to Tom’s flat. After a long conversation about how far they’d come in their friendship, they realized how essential they were to one another’s lives.

From that night on, they were a couple.

“There was one night when we had just moved in together in 2012, and we were sitting on the floor eating pizza in this empty apartment, and I looked over at him and thought that this is exactly where I want to be,” Harriet says. “Not necessarily always eating pizza on a floor, but with him. It finally felt right.”

Her schedule as a flight attendant was still hectic, with a set pattern of “three days together, three days missing one another,” Harriet says. But they learned to extend their time together with Tom always picking her up from the airport and sometimes hopping on a flight with her to keep her company.

“Her friendship has been so important to me and my life,” says Tom, who owns an investment company in London. “So now having this relationship and future built on that feels like a dream.”

Soon they were talking marriage, and in July 2013, Tom proposed on an island off the coast of Portugal.

On June 28, Harriet McKechnie, 32, and Tom O’Mahony, 31, wed on the beach where they met more than 15 years earlier. This time, there were no airborne bottle caps to be found.