Two years ago this summer, former Camden-Rockport Middle School chums Mariah Klapatch and Josh Gerritsen were swimming in Megunticook Lake when they decided to make a horror movie. The suggestion came from Gerritsen, whose best-selling author mother had already proposed the same. On Saturday night, Oct. 29, the result will be screened at the Camden Opera House.

“I knew we were going to come this far and I had a feeling we were going to make a good movie,” Josh Gerritsen said earlier in the month, before an in-progress screening at Klapatch’s mother’s house.

Klapatch wanted to make sure her Pearl Street neighbor Dot Marchant, who was about to leave for her warmer winter home, got a chance to see the film. Marchant is among the handful of Camden and Rockport homeowners who gave the filmmakers access for “Island Zero,” penned by medical thriller novelist Tess Gerritsen, who grew up on old-school monster movies.

Other locations include the Swan House Bed and Breakfast in Camden and Victorian by the Sea in Lincolnville; Pen Bay’s Picker Family Resource Center; Camden and Rockport harbors; Duck Trap; Durkee's General Store on Islesboro; the Islesboro Ferry Terminal, and all around the island; and Monhegan.

“I love the idea of an ‘Island Zero’ tour. It’s silly, but I would love it,” said Klapatch, who is based in Los Angeles. “The deluxe tour would go to Islesboro; the premium deluxe tour goes to Monhegan.”

There was no principal photography shot on Monhegan, but it has a brief starring role.

“At the very beginning of the movie, you’ll see Monhegan, but you won’t recognize it, because we digitally removed Manana,” the island next to Monhegan, Klapatch said.

The result is the fictional Tucker Island, where the contemporary story takes place around Christmas time. The shoot took place during a few cold and damp weeks in March, and the first location was Marchant’s house, standing in for the island home of “Ruth and Alvis.” The elderly couple are played by Boston-based Anabel Graetz and Waterville’s Richard Sewell.

“I wasn’t here, I was in Florida, so I have to see what they did! But they’re the best neighbors I’ve ever had,” said Marchant, who has known Klapatch since she was an infant.

That neighborly connection goes way back; Klapatch’s mother was 8 when the family moved into the house. So Klapatch, the movie’s producer, gave the cast and crew firm instructions the night before filming about treating Marchant’s home “like it was the Louvre!” Day 1 dawned a rainy mess, so it took a lot of cardboard and attention to keep the wet and mud outside.

“It ended up being a really good thing, because we had this big challenge on our first day and everyone functioned so impressively,” said Klapatch. “It really set the tone for the rest of the shoot.”

Any day on a film shoot has the unexpected, in Klapatch’s experience, but that chilly, soggy beginning presented a challenge no one had foreseen. Marchant had warned that the heat and water were shut off, but that only seemed to be a comfort issue. When they started shooting, however, the actors’ breath “steamed,” something very hard to remove digitally, even if a project has the budget for it — and “Island Zero” didn’t.

“We had our own generator, but needed the power for the lights. So in-between shots, we would turn out some lights and turn on the space heater,” said Klapatch.

The day, however, was just too cold for that stopgap measure. So the crew got creative.

“We got cups of ice and before the actors said their lines, they had to suck on ice,” said Klapatch.

“It kind of worked — a little bit,” said Gerritsen, who directed the film.

“If you watch really carefully, you might see a little steam,” Klapatch said.

What anyone watching the film certainly will see are a lot of familiar faces, particularly in the early scenes. Those playing residents of Tucker Island include Terry Bregy, Mary O’Brien, Nancy Nickerson, Robin Jones and the producer’s father, Joseph Klapatch, as well as Maine actors Stephanie Atkinson, Joanna Clarke and Ann Gravél. Everyman Rep’s Paul Hodgson has a cameo scene; and Camden Hills Regional High School freshman Elaine Landry is among the independent film’s principals.

In addition to Graetz, three other professional screen actors were brought in for the Midcoast film. Male lead Adam Wade McLaughlin was on HBO’s “Vinyl” and is a recurring character on Showtime’s “Billions.”

“He’s in a bunch of TV stuff. I don’t think he’s really ‘hit it’ yet, but he’s super-talented,” said Klapatch. “The two other leads we brought in from out of town are both doing really well, but don’t have one thing that they’re known for,” she said in regard to Laila Robins and Teri Reeves.

And then there is the New-York based professional who once played the Camden Opera House stage, starring in, among other things, Camden Civic Theatre’s “The Who’s Tommy.” Klapatch said the project was having a hard time casting a particular part she had hoped to do locally.

“It’s sort of a studly guy, but it’s also a really big part, so the actor had to be really good,” she said. “And I was talking to my dad one night and he was like, well, what about Matt Wilkas? And I was like, whaaaaat?! He’s so perfect for it!”

His casting not only fit the bill, it also gave Wilkas, whose credits include the lead in the SXSW-premiered film “Gayby,” a West Coast-hometown connection.

“He’s actually in L.A. right now, driving my car! He’s doing a workshop with iO West, an improv group out there, for a few months,” Klapatch said.

Wilkas is dating an Olympic skier, who came up for the shoot, she added. Camden Hills student Jade Hazzard was the film’s intern and is making a behind-the-scenes documentary about the project. While the Screen Actors Guild actors are not included, “There’s this random famous athlete in her student film!” Klapatch said.

The actors were costumed by Belfast’s Elaine Bielenberg; and Angela Bonacasa, fight coordinator for many a Midcoast production, served as same on “Island Zero.” In fact, most of the crew is Maine-based, a goal from the beginning. The movie is executive produced by Josh’s parents, Tess and Jacob Gerritsen, of Camden, both of whom make an appearance on camera … sort of. The retired medical doctors have a hands-on scene each, those hands being covered by surgical gloves.

“They’re really pleased with it,” said Josh Gerritsen, who had screened the film earlier in the day at his parents’ home. The evening’s screening would be the first time he’d watch “Island Zero” with people who hadn’t seen it.

“I’ve lost all perspective on whether the movie’s scary or not,” he said. “I hear it is!”

No worries there — “Island Zero” is a monster movie of the classic three-quarters “gonna getcha” to one-quarter “gotcha!” ratio. The latter’s moments were yet to get their digital effects this night, but the tense storytelling was well established.

“We got lucky. Our editor also does some visual effects, so he did some of them [as did one of the cameramen]. And we also worked with a visual effects company in Baltimore called The Garden,” said Klapatch.

There, they had some more luck. The Garden Creative LLC typically does not take on clients of “Island Zero” size, “but they have a weak spot for indie horror movies and decided to help us out,” Klapatch said.

With credits still to be added, the film ran about 97 minutes, which should just bring it to Klapatch’s ideal.

“My thing is, I don’t like a move to be over 99 minutes, unless it’s super-serious,” she said. “We definitely cut a lot! But I feel it keeps the movie crisp and entertaining.”

One cut was a tough call for both Klapatch and Gerritsen. Gravél, “kind of like a Maine scream queen or horror maven,” is in two scenes but used to have a third. Gravél is a veteran of a number of the Damnationland short films and has directed her first, included in this year’s tour (as is the “Island Zero” trailer).

“We love her! There’s this character who was written primarily as kind of a functional tool for the plot, but when we cast her, we added some things here and there,” said Klapatch. “We had to cut out one scene, but there’s still some little fun stuff in there for her.”

Klapatch and Gerritsen found themselves in their film, as a matter of necessity, when a scene “needed warm bodies.” As cold as that first day’s shoot was, the final bit of principal photography was hot — really hot. Shot on May 1, it involved burning down a house on Thomas Street, after it had been used for firefighter training.

Doors will open at 6:30 p.m. Saturday for the 7 p.m. screening which, due to festival submission requirements, is by invitation only. Getting an invite is easy, as long as seats remain; just send an email to Klapatch via Admission is free, but donations to the Camden Fire Department and Camden Opera House are encouraged. NOTE: AS OF OCT. 26, HOUSE IS FULL; THERE IS A WAIT LIST.

“It's our way of saying thank you to these two organizations for being so generous to our project,” said Klapatch.

The one-time screening will be the last chance to see “Island Zero” for a while. The team has already applied to be in the Sundance and Slamdance film festivals and will be submitting to more, including the Halloween and horror-themed fests whose deadlines were just missed this year.

“Toronto has a really excellent horror fest. Fantasia fest, the Telluride Horror Show and Fantastic Fest are the three biggest,” Klapatch said.

Applying for festivals is a lot easier than it was in the pre-digital age. Klapatch said there is a website partnered with Vimeo that makes the process quite simple.

“It took me like three minutes to do our Slamdance submission the other day,” she said. “They do cost money, so we’re not going to apply to 45 horror festivals; it probably will be more like 10.”

The production company credited with “Island Zero” is Donkey Universe Films, which is appropriate. The genesis of the project occurred while Josh and his mother were working in the garden of his pasture-based farm in Lincolnville.

“It’s the whole brand,” he said. “You can watch the Donkey Universe film and then buy the eggs from the farm!”

For more information on the film project, check out the Island Zero Movie page on Facebook.