GARETH EDWARDS knew the power of the green screen, the blank matte used by special-effects artists to create computer-generated imagery. But he rejected it for his new science-fiction movie, “Monsters.” He learned from experience: A special-effects engineer with a background in television, Mr. Edwards once constructed a massive green screen to manufacture battles digitally in the hourlong BBC movie “Attila the Hun,” which he also directed. The vast crew needed, though, quickly overwhelmed him.

He said: “I went: ‘Wait a minute. There’s 50 of you. We’re paying thousands an hour to make life easier, and all we’re doing is relentlessly saying we can’t do anything because it’s too expensive or will take too long.’ ”

A 35-year-old Briton with a teenager’s energy, Mr. Edwards decided to think small for “Monsters,” which he originally envisioned as the record of an alien invasion captured on a survivor’s camcorder. Then a friend sent him the trailer for the 2008 low-budget “Cloverfield,” which has a similar narrative strategy. “What gutted me the most was that it was from some major studio,” he said. “I wouldn’t have minded if it had been some kids with a camera who had gone off and rewritten the rule book on how you make movies.”

Image Mr. McNairy in another scene from this low-budget film. Credit... Magnet Releasing

With “Monsters,” opening Oct. 29, Mr. Edwards intends to do just that. Containing 250 effects shots produced on his laptop and completed with a budget estimated at $500,000, the movie presents a radical counterexample to costly Hollywood productions. Set six years after hulking, animalistic extraterrestrials have crash-landed in America, “Monsters” follows a war photographer (Scoot McNairy) as he accompanies his employer’s daughter (Whitney Able) through a quarantined area in Mexico, near the United States border. Emphasizing the budding romance between the leads, the plot is a genre hybrid that Mr. Edwards jokingly described as “not a monster movie for girls or a love story for boys,” but “a road movie for aliens.”