“Good Boy” is elemental horror — a tale of a sick man under natural and supernatural assault — and his dog.

And the marvel of this movie is the performance by that real live dog. Director Ben Leonberg picked up on his pet Indy’s “range” of expressions and emotions and trained, tricked and puzzled the Novia Scotia Duck Tolling Retriever into a canine performance — created by editing — for the ages.

Indy is so devoted to his owner, Todd, that he’s never distracted by whatever goes on beyond the camera’s field of view during a movie shoot. His eyes don’t leave Todd (Shane Jensen) in his moments of need, peril and confusion. But Indy is wary of grandpa’s “haunted house” in the country. Indy wanders. Indy sees and hears things that intrigue and alarm him.

“Good Boy” is a simple, scary thriller that leans on one of the classic tropes of the genre — “The dog knows.” And seeing what Indy sees and taking in what Indy’s frightened, confused but loving and loyal eyes absorb makes us fear for what he doesn’t understand, what faces his master and could face him as well because he just doesn’t know better.

Todd, whose face is rarely seen and then not until the third act, is sick. We gather this from calls from his concerned sister Vera (Arielle Friedman), and from her visit, when she finds her brother coughing up blood.

Todd’s response to this is to flee to grandpa’s old house on the farm. He walks Indy, fusses over him. And when his medical situation turns dire, Indy is with him when he goes to get a cat scan and some very bad news.

But Indy is picking up other threats from their new surroundings. It’s not just their hunting, camo-nut, foxtrap-setting neighbor (Stuart Rudin) that they have to be wary of. There are noises, shadowy figures in the background Indy doesn’t see. And grandpa (horror icon Larry Fessenden), seen on old vhs tapes, had something weird going on around him.

Indy hears whimpers from a locked cellar, which can only be the ghost of Bandit, grandpa’s long-dead dog. He even sees him.

The tangible “threats” here come from the usual horrific visuals — gnarled, skinny, twig-thin black fingers, menacing eyes that pierce the dark, unseen evil reaching from beyond for somebody and somebody’s dog.

Leonberg wisely lets his beautiful, expressive dog’s face tell this story and sell this terror. This low-budget jewel all but mocks every film production that needs a dog but which uses a CGI one instead of a living, breathing, loving and relatable pet, from “Call of the Wild” to “Superman.”

There is no substitute for the real thing, and even people who don’t love dogs reognize that.

The tropes trot by as blood and black bile reach out from beyond — or maybe just from Todd’s diseased condition — and the dog takes in his master’s suggesting horror choices for TV viewing (“Carnival of Souls”) on his grandpa’s old TV.

The film’s limited dialogue and dogs-eye-and-ear-view (muffled dialogue, unseen faces) give its story an underexplained mystery, which works to its advantage. The pools of darkness, gloomy exteriors and shadows underscore the “less is more” ethos that the production lives by.

But as Spielberg himself could tell you, if there’s no Indy, there’s no movie. W.C. Fields may have warned his fellow actors to never co-star with “children or dogs.” “Good Boy” makes the humans all but superfluous as its star delivers some of the most realistic reactions to the unexplainable this time-worn genre has ever seen.

Rating: PG-13, bloody violence, profanity

Cast: Indy, Shane Jensen, Arielle Friedman, Stuart Rudin and Larry Fessenden.

Credits: Directed by Ben Leonberg, scripted by Alex Cannon and Ben Leonberg. An IFC release.

Running time: 1:13