WATCH: Flash Mob Hands Out Flowers for a Super-Sweet Marriage Proposal

When Boston-resident Jack Cushman decided to propose to Teresa Elsey, his girlfriend of nearly three years, he was certain that she'd say yes. So all that was left to figure out was how he'd make the moment memorable.

"Since I wasn't too nervous about the question itself, I decided to come up with a ridiculously complicated plan to worry about instead," Cushman wrote in an essay for The Huffington Post.

"My first thought was just to get a bunch of strangers to give Teresa flowers as she was coming home from work, people in all kinds of improbable places," he told Yahoo! Shine in an interview. "I wanted to create a world where good things happen for no reason, a world full of bounty. I just loved the idea of just being able to give that to her."

Related: Why do people propose in public?

When she suggested a date to the Institute of Contemporary Art (ICA) in Boston, where they'd had their first date back in 2009, the timing seemed perfect. He enlisted his best friend and his brothers to help him pull off the proposal.

"They've been together for so long that it wasn't surprising that they were getting married, but he wanted to do it in as surprising way as possible," his friend Ben Pender-Cudlip, a documentary filmmaker and the founder of Unrendered Films, told Yahoo! Shine in an interview.

Inspired by a music video project they'd done together, Cushman decided to organize hundreds of strangers into a flash mob to give his girlfriend 300 pink and white flowers as they walked from her office to the museum on August 2. And he had his film-making friends document the whole thing.

Jack Cushman proposes to his girlfriend of nearly three years, Teresa Elsey, after a flash …"Ben and I had worked on video projects before, so we weren't going to do something this crazy without videotaping it," he told Yahoo! Shine. "We thought it would be for our friends, not that we'd let so many people in on it and share it with them as well. But it became so clear when people were watching it [the proposal] that they were part of it. It was completely unexpected. That was the amazing thing."

The video has a fun, special-ops feel that lets viewers in on the excitement: Code names and walkie talkie-like communication are mixed in with shots from multiple hidden cameras on the ground and in buildings along the route. Cushman, who moonlights at Unrendered Films as a developer and director when he's not working as a web programmer and appellate attorney, coordinated the plan. His brothers prepped the crowd at the ICA, asking them to participate and warning them not to give away the secret when they saw Elsey. Pender-Cudlip led the team of four cameramen including one stationed on the 14th floor of a nearby high-rise, directed the video, and then stayed up all night editing it so that the couple's parents could watch the proposal play out online the next morning. ("My mom was laughing and crying," Cushman says. "My Dad was just speechless. And he's never speechless.")

Cushman was outfitted with not one, but two microphones -- a wireless one taped to his chest, and backup that fed into a recording device in his pocket, Pender-Cudlip says -- and the guys enlisted friends at Elsey's office to let them know when she left the building.

"We had someone tail her with a GPS, and had them take video of her and coordinate with flower-givers along the route," Pender-Cudlip says.

Related: 5 fabulous flash mobs (one's a wedding!)

Elsey turned down the first stranger who offered her a flower, but accepted a few others as she walked with Cushman to the museum, about a mile away. "Teresa guessed that it could be some kind of fraternity stunt," Cushman says, but didn't think he had a part in it. As they got closer to the museum and more and more people handed her flowers, "She started to think that I was planning something, but didn't know what it was."

When they got to the ICA, where a free concert was in progress, "she was surrounded by people offering her flowers," Cushman says. He slipped away during the commotion to hustle into a tuxedo -- he'd cut it up and velcro'd it back together earlier in the week, to make for a quick clothing change -- and by the time the crowd of finished handing her carnations, Elsey's arms were overflowing with flowers. He led her to a nearby bench, knelt in front of her with a microphone, and asked: "Teresa Elsey… would you marry me?"

"Of course!" She replied, beaming. The crowd cheered.

"After the proposal I had planned to whisk her away, because she's the kind of person who would want privacy after something like that. So we made our grand exit," Cushman writes in his essay. "The video kindly cuts out before I realized we were walking the wrong way and we snuck back through the cafe."

"We got into a waiting car out front and went out to a wonderful, hours-long dinner, where we mostly just kept staring at each other and laughing in disbelief," he continues. "When we got home, my co-conspirators had wound Christmas lights and flowers around the spiral staircase leading up to our bedroom. We found out later that they were hiding behind a tree when the cab pulled up to drop us off."

Cushman credits a team of more than 30 friends and businesses for helping him pull off his over-the-top proposal. (They also gathered sweet marriage-proposal stories from other people in the crowd; you can watch those videos here.) The couple hasn't set a date yet, but Cushman says the wedding will probably take place sometime next summer.

"The engagement itself has brought us joy beyond words," Cushman writes. "But it's also been such a gift to share our joy with family members and friends and strangers -- to know that other people are joining us in that moment, and somehow believing in the crazy project we're undertaking. It feels like something we'll have to spend our lives living up to."

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