Warning: This post contains spoilers for Undertone, now in theaters.

From the moment Undertone opens with an eerie rendition of "Baa, Baa, Black Sheep" echoing through the sterile rooms of an oppressively quiet home, it's clear the sound-fueled horror film isn't going to waste any time playing up its central gimmick: auditory terror. The classic nursery rhyme is being sung by a young woman named Evy Babic (Nina Kiri) to her terminally ill mother (Michèle Duquet), who lies wordlessly in bed in what a hospice nurse warns Evy over the phone are likely the last days of her life. Evy has been left to care for her comatose mother alone and spends her days cleaning her up, pleading with her to eat and drink, and sitting beside her upstairs as she awaits her eventual "death rattle."

To distract herself from the grim task at hand, Evy devotes her nights to recording her popular paranormal podcast "The Undertone" with her London-based co-host Justin (Adam DiMarco), who we hear over the phone but never see. In fact, we never see anyone besides Evy and her "Mama," as the film is shot entirely within the two-story childhood home of writer-director Ian Tuason in an effort to capture the claustrophobic and isolating environment Tuason experienced while caring for his own terminally ill parents.

“For that whole time, I felt trapped in my childhood home. I was drinking to numb the stress I was under—but I was also writing this movie the entire time. The character that became Evy Babic in Undertone was me," he said in film's production notes. “I always considered my mother’s company as the safest place I could be, and when I saw her in this vulnerable, dependent condition at the end of life, my imagination took off."

As creepypasta enthusiast Tuason's debut feature opens, the "Undertone" co-hosts decide the topic of their next episode will be a mysterious email Justin received containing 10 audio files to be played in order. Ignoring concerns most people might have over whether the files are spam or inappropriate or simply boring, they decide to listen to them for the first time live on air and see what happens. One by one, they hit play on the recordings, which are revealed to have apparently been made by a pregnant couple whose lives were being infiltrated by a malevolent force. While this plays out over the course of three podcasting sessions, Evy learns she herself is unexpectedly pregnant—a development she is less than enthused about—and begins to notice strange and increasingly ominous happenings that seem to mirror the haunting occurrences she's heard in the recordings. Despite her Catholic upbringing, Evy is a paranormal skeptic and religious nonbeliever. But as the horrors of the files slowly seep into her reality, she finds herself starting to doubt her doubts.

The slow-burn terror of Undertone lies in both its immersive soundscape and clever cinematography, as the latter compels viewers to constantly be searching the dark corners of the frame to see whether anyone (or anything) is lurking in the background. Usually, they aren't. But when they are, it certainly sends a dread-inducing chill down your spine—especially once Evy's immobile Mama appears to start moving about the house. 

How does Undertone end?

To cut a long story short, by listening to the recordings, Evy has opened herself up to female demon named Abyzou that is known to possess pregnant women and cause miscarriages and infant mortality out of intense jealousy brought on by her own infertility.

"I wanted to find a demon from ancient folklore that killed children. The idea was that Evy doesn’t want to have a child, and maybe she’s unconsciously summoning this demon to take away this burden that she has. That was the original idea. And then I found Abyzou, which was a demon from the Testament of Solomon, which is an ancient book, an ancient scripture. She was perfect for this movie. Again, it’s just like nursery rhymes. The Testament of Solomon is passed on through oral tradition," Tuason told In Review Online. "Solomon’s Testament was his oral rendition of what he says he went through. Then that gets passed on from generation to generation until we have this being that exists only in our imagination. But since we’re all connected in many ways, in language and culture, who’s to say that this entity doesn’t really exist?"

By the time the final recording is playing, it's become too late to turn back, and Evy is plunged into a nightmare of her own making as Abyzou has apparently used the files as a channel to enter the house. However, in its final moments, Undertone cuts to black, leaving audiences to decide what's happening to Evy and her mother based solely on what they're hearing. We hear screaming, and Evy calling out to Mama, and what sounds like a struggle that ends with at least one person falling down the stairs. Are both Evy and Mama dead? Did a possessed Mama kill her own daughter? Did Evy trip down the stairs in the midst of a psychotic break?

It's up to your imagination to fill in the unseen horrors. And as we all know, the frights our imaginations conjure up are often far worse than anything a movie could actually show us.