Directed by Swedish filmmaker Frida Kempff, Knocking holds an uncomplicated kind of horror premise: A woman named Molly (Cecilia Milocco) moves into a new apartment and hears a continuous knocking sound.

While certainly irritating at the start, things become more and more concerning as the knocking becomes far too constant and is eventually accompanied by other, more disturbing noises. Naturally, no one else in Molly's apartment complex seems to hear any of it, nor do they believe her idea that the sounds might mean someone needs help.

That's the entire plot of this Sundance Festival favorite, something that might stretch itself far too thin if it weren't for the wise decision to keep the film at a tight 78 minutes. And what Kempff, Milocco, screenwriter Emma Broström and phenomenal cinematographer Hannes Krantz (along with the rest of the cast and crew, of course) are able to do within that crisp runtime is wondrous - Knocking is a near masterclass in staging a simplistic yet appropriately tense psychological thriller that continues to build onto itself.

It's a story about who owns the truth and whose voice matters, one that clearly deals with the sad universal truth of how women struggle to be believed on serious issues and are often ignored and gaslit. The film also plays with what happens when we stop helping each other in general; that sense of apathy that seems to permeate within far too many people.

Molly is shown to be a good person, her empathy and desire to help someone she believes to be in peril continuously overrides her feelings on how everyone around her perceives her, or even what they're telling her. But that goodness is also shown to backfire on her, like when she calls the police at one point only for things to quickly go south because of it.

Of course, part of Knocking's brilliance is in how, to further add to the disorienting and claustrophobic feeling of the film, it makes Molly somewhat of an unreliable narrator. Molly, you see, has just been released from a stint in a psychiatric facility. After a traumatizing personal loss, she had a mental breakdown and has only recently rejoined society, but visions of what she experienced still continue to haunt her.

So throughout Molly's ordeal, you can never really be sure whether it's really happening or if it actually is all in her head like everyone tells her. Krantz's striking camera work, which employs fascinating and squeezed angles to better capture Molly's rapidly deteriorating state of mind, keeps ramping up the more her sanity starts to slip. Alongside Milocco's performance, it makes for the film's strongest aspect.

There are multiple layers that keep piling onto Molly's plight: a ticking clock, a sweltering heat wave (a real stroke of genius there) that you can almost feel coming off the screen, and in a jolt of surprise body horror, she also develops a nervous tic that makes you want to look away.

She can't sleep, she's roasting in her apartment every day and night, she's finding terrifying things like spots of blood or a message in the elevator, everything coalesces to make for a nightmarish fever dream. Knocking goes from a hazy and ruminating wander through a weary and grieving mind to a full-blown horror movie once the titular noise begins, and it never takes its foot off the gas after. It's sharp and concise paranoia and madness at its finest, and its scare factor isn't something to be underestimated.

'Knocking' is now playing in theaters and is available on VOD.