“Luckily, we got on like a house on fire,” Macdonald said on a recent video call.

It was a perfectly uncanny environment in which to dream up the climax of “Prophet,” which reads like a Christopher Nolan movie — gray vistas, baroque plotting, a premise just this side of dorm-room philosophizing — but more madcap. Kicking off in Britain before crisscrossing the American West, the story follows the spread of a military-designed chemical called Prophet, which draws subjects into a nostalgic trance, then catatonia. But the substance keeps gaining new, disturbing properties. People who ingest it start physically conjuring objects and places from their most precious memories. Scrabble sets, teddy bears and soon an entire retro American diner spring into existence.

Enter an unlikely duo, called in by their governments to investigate: Adam, a gruff American super-soldier, and Rao, a dissolute former British intelligence officer with a talent for detecting falsehoods — unless Adam is telling them. “That’s basically ‘Twilight,’” Blaché remembers skeptically saying, early in the drafting process. Tug on one reference in this novel, and you see that it’s knitted from skeins of pop culture that its authors love: the video game “Control”; odd couples from “Peep Show” to “The X-Files”; L. Frank Baum’s Valley of Lost Things.

It’s surprising stuff coming from Macdonald, who became famous for their elegiac nature writing, particularly the best-selling memoir “H Is for Hawk.” (Macdonald and Blaché both use they/them pronouns.) Christina McLeish, a philosopher and one of Macdonald’s oldest friends, recalls that at book events, readers would ask what was coming up next. Macdonald would deadpan, “I’m going to write a gay sci-fi romance novel.” Everyone laughed it off.

But Macdonald has always nurtured their inner nerd — the kid flipping through Isaac Asimov and Arthur C. Clarke paperbacks and semi-psychedelic magazines — even as they focused on nonfiction. Macdonald’s next project was supposed to involve travel to the Midway Atoll, to research the writing of “a huge book about albatrosses and the Navy, the world, environmental guilt, you know, stuff like that.” Then the pandemic scuttled those plans, leaving Macdonald stuck at home in a Suffolk cottage, alone save for their parrots and the internet.

“I was kind of losing my mind a bit. I wasn’t writing. I couldn’t get out really, or do much nature stuff,” Macdonald said. “And I just started talking to Sin.”

At the time, Blaché was working a retail job they hated at a local adult shop. Lockdown freed up time for walks and music — Blaché sings and plays banjo, violin, piano, various folk instruments and, “like everybody else who was alive in 2010, the ukulele” — and for online chats with Macdonald. The two have known each other since roughly 2009.

“Everybody in my Twitter circles at the time was really popping off on ‘Doctor Who,’” Blaché recalled of their early friendship, cradling a mug of tea on a recent video call. “Everybody was just waist-deep in ‘Doctor Who,’ and — ”

“We got involved in the discourse,” Macdonald nodded from an adjacent window in the chat.

For years at a time, the two didn’t talk much — just floated companionably in the eddies of their shared enthusiasms. (At one point, Macdonald did mail Blaché a Star Wars T-shirt: “It was really aggressively kind,” Blaché said.) The friendship truly bloomed during the pandemic.

“Everything was so isolated and so miserable and dark,” Macdonald said, “and we talked, I think in response to that, about things that we loved — video games, sci-fi, movies. And these conversations ended up being a lot about nostalgia.”

Both were fascinated by the concept. “Nostalgia is a weird thing when you can’t really hold onto actual things,” Blaché said, recounting a childhood in which their family moved from Los Angeles to Dublin to the countryside, enrolling them in Irish-speaking schools. Blaché grew up feeling distant from the cowboys-and-blue-jeans notion of American culture but also what their family jokingly called “barefoot Ireland”: the tendency to romanticize an earlier, simpler (if also impoverished) era. Meanwhile, Macdonald’s writing, often on the themes of grief and the environment, meant that they constantly contended with the longing for an irrecoverable past.

In the fall of 2020, Macdonald asked Blaché about co-writing what they thought of as a novella. Macdonald had a mental image of an American diner in the middle of a sugar beet field and a premise about the military weaponizing memories. Would Blaché help with characters and dialogue?

The two swapped snippets of prose via direct messages on Twitter and soon abandoned the strict division of labor they’d pictured at the outset — though Macdonald remained stronger at atmosphere and exposition and Blaché at dialogue. (Macdonald never quite got the hang of American slang.)

Through online pursuits like National Novel Writing Month, Blaché was accustomed to thinking of storytelling as shared play. Macdonald experienced more culture shock co-writing with someone, but came to find the back and forth addictive: “I never knew what was going to turn up the next day. And then these magical things happened.”

Blaché and Macdonald chatted for hours — usually over social media, sometimes over WhatsApp — about the narrative’s beats, various research rabbit holes and their favorite tropes. The pandemic had intensified their shared craving for escapist entertainment, said Blaché: “Fan works in general were a huge part of that.”

Macdonald, a longtime Star Wars fan, has written fan fiction under a pen name. “I just thought it was phenomenally lovely that these stories that didn’t cost anything, and were genuinely written from love, were all over the internet,” they said. “Joyous, creative, many of them collaborations.” Blaché, who at the time was going through a “Final Fantasy” phase, suspects that almost everyone has a private fan fiction habit: “maybe not Salman Rushdie,” they allowed, “but I’m pretty sure most people have this secret little AO3 [Archive of Our Own] thing.”

The central romance of “Prophet” is instantly clockable as a fan fiction mainstay: Adam is outwardly stoic but achingly sensitive; Rao is your classic manic pixie dream Brit, fey and hedonistic. There’s a thwarted kiss, many missed signals and a delightfully ludicrous scenario forcing them together, involving “eight weeks of mission-mandated shared hotel rooms.”

The prose, too, is characteristic of spaces like AO3 and Tumblr. Written in the close third person, the scenes in “Prophet” are banter-heavy, studded with descriptions of gestures and mannerisms: a head tilting, the lift of a single brow, significant looks. The language revs up into sugar-rush run-ons about the characters’ emotional states, punctuating their realizations with a single italicized word: “Oh.”

The authors didn’t shy away from this: “I wanted it to be fan fiction of something nobody had seen before,” Blaché said, “like these characters were loved before you even knew what you were reading.”

“Prophet” delivers the fizzy, immaculately engineered satisfaction of a can of Coke, but with an almost metallic aftertaste — like its pleasures shouldn’t be trusted. That reflects the authors’ ambivalence about recycled content and familiar comforts: In “Prophet,” characters’ encounters with childhood totems become nightmarish and literally all-consuming. Looming over our fuzzy memories of Saturday morning cartoons and cereal is the specter of exploitation — a Facebook feature that pulls up old photos for engagement clicks, said Blaché, or “Disney regurgitating its intellectual property.” The continual return of past pop culture, in newer and slicker forms, completely un-killable, starts to feel menacing.

Bill Clegg, Macdonald’s agent, initially viewed the project as a lark — “a way to write and have fun of any kind in a time that was decidedly not fun,” he said over email. But when he read the pages, it became instantly clear that the project “was becoming something I was going to have to take very seriously.”

“Prophet” comes out of a digital subculture where the creative drive is almost innately social: In fan spaces, people write to delight their closest (if often pseudonymous) friends. The novel runs on that energy — heady, unembarrassed joy; the dork sublime — and, in particular, the high of finding a kindred spirit. This novel was always inside Macdonald, McLeish said — “These things have been part of Helen’s brain for years” — but it would never have been realized without Blaché. “I absolutely love the fact that they and Sin discovered this twin love of these tropes.”

The co-creators deeply love Adam and Rao, fondly nicknaming them “our terrible men” and printing T-shirts with inside jokes about them.

“Making one’s own kind of fun merchandise for a book that one is writing is a little bit up oneself,” Macdonald admitted, “but we were just having a blast.”

Blaché added, “Because there’s the two of us, it doesn’t seem so egotistical.”

Even as Blaché and Macdonald want audiences to engage with the darker ideas underpinning the plot, they also hope to turn readers into fans — and, Blaché said, they hope the art that fans make depicting Adam and Rao will be “filthy.”

“We made them,” said Macdonald, “but they didn’t belong to us.”