“Like Indiana Jones on acid.” That’s what Australian director Julius Avery wrote in the margins of the script “Overlord” when he first read it. That’s because the story begins as a pretty standard World War II flick and morphs into a wild, monster-horror film — a rare WWII-Zombie feature film mashup.

“This isn’t your average war movie,” Avery tells The Post from Los Angeles. The second half is “bonkers, completely bonkers,” he says. “There’s probably nothing else like it out there.”

The film is set the night before D-Day, which in real-life had the code name Operation Overlord. The mission of a group of American paratroopers is to blow up a radio transmitter that Nazis put atop a church. There, however, the soldiers discover a grim research lab where Nazi scientists are resurrecting dead bodies, via an injection, to create a 1,000-year army.

To make this unlikely mashup more believable — or at least allow the audience to suspend disbelief — Avery focused on creating a personal connection to the American soldiers and making war scenes that could have come out of any classic war film.

“Everything is pretty real, which makes it even more scary,” he says. “I took a lot of time balancing the emotions and the action and the crazy horrifying stuff. You gotta love the characters before they start going to hell.”

Avery, whose first film, “Son of a Gun,” started off as a standard-issue prison film, veered into a classic crime thriller. “I’m a big fan of mixing genres,” the director says. “I want to have that surprise.”

The director says he’s been a war-film buff since he was a kid because his grandfather was a veteran of the Allies’ North African campaign during WWII. “I used to look through his photo albums as a young boy, and he would tell me about his adventures and show me his medals. I could tell it was a part of something big.”

For “Overlord,” Avery tried to stay away from excessive use of CGI. The actors playing zombies had five hours in the makeup chair to get their creepy looks. And that enabled the regular-soldier actors to react viscerally to the creatures, something harder to do while acting opposite a green screen.

“I love old-school horror films,” Avery says. “There’s a texture and a feeling you can’t replicate with CGI.”

Avery learned a lot from “Overlord” producer J.J. Abrams, who also works on HBO’s sci-fi-Western combo “Westworld.” “He let me take risks” Avery says. “Once J.J. hires someone he really trusts them. He oversees everything in a way that makes you feel like you know what you’re doing and you’re in control.”

Despite Avery’s admitted love of mixing movie genres, he acknowledges that it’s not always a sure thing. What mashups shouldn’t we see?

“Smurfs, the little blue people, and vampires,” he says.