By ERIC KOHN

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 TV, Mr. Edwards ran through time and money on “Attila the Hun,” a BBC movie he directed.

A 35-year-old Briton with a teenager’s energy, Mr. Edwards decided to think small for “Monsters. ” C ompleted with a budget estimated at $500,000, it presents a radical counter-example to costly Hollywood productions. Set six years after 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.

Mr. Edwards described it as “not a monster movie for girls or a love story for boys,” but “a road movie for aliens.” The third world setting has invited comparisons to “District 9,” in which the persecution of aliens alludes to apartheid.

But “District 9” partly owes its special effects to the sizable team of Weta Digital. The effects in “Monsters” were Mr. Edwards’s alone. This wasn’t an entirely new feat for Mr. Edwards, whose previous work includes depictions of apocalyptic tornadoes and solar storms for the American series “Perfect Disaster.” After “Attila,” Mr. Edwards said, “I was hunting for a cheap but potentially big-scale project that could use visual effects.” During a three-month shoot in Mexico and South America, Mr. Edwards worked on “Monsters” with two laptops .

His editor, Colin Goudie, assembled the movie on one computer, while Mr. Edwards created the effects on the other. “Usually trying to get a visual effects artist to put on screen what you and the director want is like pulling teeth,” Mr. Goudie said. “With Gareth, because you were dealing with the person who actually did the effects rather than the effects supervisor, it led to a much more creative environment.

On the set the actors improvised dialogue while Mr. Edwards conceived effects . The extensive post-production took place at Mr. Edwards’s home in London. There he designing the tentacled bioluminescent aliens .

“Monsters,” distributed by Magnolia Pictures, will be released around the world through autumn and winter. Tom Quinn, Magnolia’s senior vice president for acquisitions , said, “I’ve shown it to people who produce films with $2 million dollar budgets, and they thought it cost way more.”

James Richardson, a founder of the British production company Vertigo Films, sees potential in the “Monsters” model: “People who can create magic on their computers with a sense of visual storytelling can now make movies by themselves.

” Mr. Edwards cautioned against the blockbuster proclivity to think big, insisting that special effects must represent a singular vision. “The term ‘computer generated imagery’ suggests that computers generate the stuff,” he said, but people do. “That’s why they’re good.”