TORONTO—When a science-fiction thriller features a wall that keeps monsters from Mexico out of the U.S., you might think that aspect must have been the film’s starting point. But for first-time British filmmaker Gareth Edwards, the creepy creatures came before the political allegory when he dreamed up Monsters (which opens on Friday [November 12] ). Once he’d created his creatures, Edwards set out to make “the most realistic monster movie in history”.

Edwards, an award-winning visual-effects creator, was “on holiday” in the Maldives Islands and found himself fascinated by some fishermen hauling in a net. “When you do visual effects for a living, you never really stop and switch off and relax. So no matter what’s going on, I’m always picturing spaceships or robots or whatever, and these guys were struggling with the net and teasing each other, and I couldn’t understand what they were saying ’cause it was a foreign language. So I started thinking, ”˜I wonder what’s on the end of this net?’ And I started picturing that it’d be really cool if it was some massive sea creature, like a giant tentacled thing.”

Talking in a downtown hotel at the 2010 Toronto International Film Festival, Edwards says that as he pictured the creature, he wondered what it would be like if, instead of freaking out, the fishermen weren’t remotely fazed by what they’d caught. And that’s when the idea hit. “Imagine setting a monster movie years after normal monster movies end, when it becomes part of everyday life.”¦It’s a world that’s set, like, six years after the creatures have come and no one cares anymore, a bit like Iraq is on the news today.”

Edwards spent the rest of his holiday trying to reimagine a world where his monsters were lethal but normal. “Like, if there was a roadblock, I would picture that there was a roadblock because there’d been a massive tank battle, and I’d picture upturned tanks and dead creatures.”

He says that to keep a realistic vibe and to work within his minimal budget (estimated in several newspaper articles at about $15,000), he cast the entire film—except for his two leads: Scoot McNairy and McNairy’s real-life girlfriend, now his wife, Whitney Able (All the Boys Love Mandy Lane)—from nonactors he met on location. McNairy plays a cynical American journalist trying to help his boss’s cranky American daughter navigate through the bureaucracy and the borders from the monster-plagued Infected Zone in northern Mexico.

Monsters was shot guerilla-style, with improvised dialogue and a crew of just four people—plus a “fixer” to deal with the locals, some of whom were “cast” in the movie just minutes before being filmed.

That style clearly worked. The movie has been nominated for six British Independent Film Awards: best film, best director, best actor, best feature-film debut by a director, and awards for technical achievement and production.

As his script evolved, the political allegory that interested Edwards was the so-called war on terror. “My initial idea was that if you somehow cut the creature in half, you’d have two creatures”¦the idea being that the more you try and destroy this thing, the bigger the problem becomes. And I feel that’s a bit like terrorism, in that the more we bomb these countries to eradicate terrorism, the more we inspire people to hate us.”

As for the political allegory that most viewers will be seeing in a world where self-proclaimed “minutemen” patrol the southern U.S. border and illegal immigrants helped swing the Senate for tea partiers, Edwards laughs. “I’d never even been to Mexico until we landed to start filming. It was not on my radar at all.”