‘The Babadook’ introduces a impressive new horror director

Before the first line of dialogue, “The Babadook” stands out on four counts.

It’s an assured debut by a filmmaker who took nine years to adapt her 2005 short “Monster.”

It won the 2014 New York Critics Circle Award for best first film, a rare achievement for horror.

It comes from Australia, which seldom sends movies to America these days and hardly ever sends us frightening films.

And it comes from writer-director Jennifer Kent, one of the few women to get theatrical distribution in this genre. It’s not sexist to say her gender may have contributed to its success; she had insights a guy might not have had.

Only two characters matter. Amelia (Essie Davis), a single mother in her early 40s, still grieves for the husband who died in a car crash en route to the hospital when she was pregnant. Sam (Noah Wiseman), who’s turning 7, exasperates his working mom: He can’t play easily with other kids, has screaming fits and even convulsions when he’s angry and has become obsessed with the idea that monsters lurk in their house.

They read a bedtime story each night, after she checks the rooms for creatures, calms her chattering boy and dismantles Sam’s monster-repelling apparatus. (He’s prepared to bludgeon, stab or shoot them with a dart.) One night, she picks up a story called “Mister Babadook,” about a shapeless creature that invades households. It seems at first to be a reassuring tale about a kid defeating evil, but it turns out otherwise.

The crux of the film gets expressed in lines from the book: “Don’t let him inside.” You’ll be scared to death when you see “what’s underneath.” When terrible things start to happen, we wonder whether those lines are literal or metaphoric. I was still in doubt (which I liked) when the final credits rolled.

Is the babadook inside the house or inside Amelia herself? Is the terrible thing that’s “underneath” the essence of the invader or the essence of the mother? Are the events we see happening in reality or in Amelia’s mind? Most of the evidence goes in one direction, but Kent lets us decide whether we’re watching a supernatural creature or a psychological breakdown like the one in Roman Polanski’s “Repulsion.”

The two actors give compelling performances as lonely people wrung out by circumstances they can’t control. Love binds them and limits them, because they can’t break out of their two-person world. (She ignores a persistent suitor, and he lashes out at playmates.)

Few male directors could’ve conveyed Amelia’s wretched state without making her seem hideous or shown Sam’s deep anger without making him unlikable. Kent has done both and left us to decide exactly what we think about this troubled, close-knit relationship.