It was Stephanie Harbeson’s 21st birthday. She’d just finished opening presents when her boyfriend, Gary Hudston, sat her down in front of his computer and told her to get ready for a surprise. The screen bore a single image: the cake icon from Portal 2. Under it was a button marked “Play.”

Five minutes later, once Harbeson had navigated through some of the game’s physics-based puzzles, the sentient computer and lovable Portal antagonist GLaDOS had a message for her.

“Stephanie Harbeson, as part of a psychological testing protocol, I am required to ask if you will do Gary Hudston the honor of becoming his wife,” GLaDOS said in her trademark monotone fashion.

“You can say no. I’m sure he’ll get over it. Eventually,” deadpanned the steely-voiced rogue artificial intelligence.

Harbeson spun around in her chair and saw Hudston on one knee, holding out a real diamond ring to match the one on the computer screen.

In an e-mail to Wired.com, she called it “the best birthday I could have ever wished for.”

“Turning around and seeing Gary on one knee just brought out all types of happy emotions,” Harbeson said. “I was too stunned to even say yes! I just mumbled and nodded and hugged him really tightly.”

“It was definitely a roller coaster of emotions,” Hudston said in an e-mail to Wired.com. “She nearly jumped out of her skin when GLaDOS said her name.”

Hudston, a graphics designer living in Preston in England’s Lancashire, says he had been planning on proposing to Harbeson for a while, but he was waiting for the perfect idea.

“I had always promised Stephy that my proposal to her would be geeky and unique,” Hudston said. He found his muse when he saw how much Harbeson enjoyed Portal 2, released to widespread acclaim earlier this year. As the two of them put their brains together on the game’s cooperative mode, the idea hit him.

“She was solving puzzles left and right, and for the first time I felt like we were playing a game together,” he said. “I knew what I needed to do almost as soon as we had completed the campaign.”

Throughout the years, gamers have come up with all sorts of geeky ways to propose to their loved ones. They’ve hacked “Will You Marry Me?” messages into games like Super Mario World and Chrono Trigger. In 2001, one programmer created a custom-made Nintendo Entertainment System game for a fellow geek’s proposal.

But Hudston’s proposal might just be the first one produced with support from the game’s developer. Hudston says he tracked down actress Ellen McLain, who voices GLaDOS, through her husband. She said she would be honored to help.

Hudston then contacted Portal 2 writer Erik Wolpaw, who asked the groom-to-be to send over a script immediately, so that the team could record some dialogue during a vocal session already scheduled with McLain the next day.

Hudston immediately went to work, pacing around the lobby of his hotel and frantically piecing together a script that he finally sent to Wolpaw at around 4 a.m.

With that obstacle out of the way, Hudston then had to figure out how to customize a Portal 2 level for his girlfriend to play through. He turned to modding-community site ThinkingWithPortals.com and asked if anybody would be interested in helping out.

“When he said Valve was included, I wanted to be part of it,” said Doug Hoogland, a level designer at Dark Artz Entertainment. Hoogland and designer Rachel van der Meer ended up completing the custom puzzles at breakneck speed. “Portal maps only take about 60 hours, 100 tops. So it’s almost nothing,” he said.

The whole project took about six weeks, during which the three-person team had to deal with what Hudston called “lots of group brainstorming and creative differences.” But once they pieced together the level and added McLain’s stellar vocals, everything clicked.

“I didn’t think that I’d ever pull it off in a million years with nothing but hope and some polite e-mails,” Hudston said.

Hudston now has a permanent companion cube in Harbeson, who said she had no idea what her boyfriend was planning.

“I basically tried to stay calm while I began to play the first level,” she said. “By the time I saw the heart appear on the floor, I was thinking ‘Oh my goodness. Is he going to propose?!’ Of course, I completely lost it when I got into the chapel room. It was definitely the best feeling.”