undertone

Evangeline "Evy" Babic is a young woman with a Catholic upbringing, living alone with her comatose mother, for whom she provides care. She and her friend Justin run The Undertone, a horror podcast where they report on supernatural occurrences, which Evy dismisses as hoaxes while Justin fully believes they are real. One day, Justin is sent an anonymous email containing a seemingly random string of letters and ten audio files, which they play on the podcast.

The files were recorded by a couple named Mike and Jessa, the latter of whom talks in her sleep, which Mike hopes to capture evidence of. In the first recording, Jessa sings "London Bridge" in her sleep, which Justin then plays backwards and insists he can hear her saying "Mike, kill all." This inspires Evy to research hidden messages in children's songs; she becomes disturbed when she discovers that playing "Baa, Baa, Black Sheep", her favorite childhood song, in reverse reveals the message "lick the blood off."

Evy later discovers that she is six weeks pregnant. At her mother's bedside, Evy informs her of the pregnancy, but then confesses that she feels unfit to be a mother. She later schedules an appointment at a local women's clinic. Playing additional recordings from Mike and Jessa reveal Jessa speaking what at first sounds like gibberish, but when played backwards reveals "come in, Abyzou." Through research, Evy and Justin learn that Abyzou is a demon in Mediterranean and European folklore who was said to cause miscarriages and drive mothers to murder their own children out of jealousy, as she herself was infertile. Evy notices increasingly strange phenomena around her, such as her still-unconscious mother moving on her own, lights flickering, and a statue of the Virgin Mary that Evy placed in the closet reappearing in other parts of the house.

In the final recording, Jessa insists she has to "warn" someone who is "listening", presumably referring to Evy. Justin attempts to respond to the sender, only for the email to be automatically sent back to him. A caller claiming to be Mike and Jessa's neighbor reveals that the couple was found dead in their home with plastic bags over their heads and crayon drawings of babies all over their walls; the autopsy revealed that Jessa was pregnant at her time of death. Another caller demands to speak to "Mary"—the name Evy previously said she had always wanted to give her hypothetical child—and does not relent when Justin insists there is nobody there by that name. A final caller named Abby begs for help in calming her incessantly crying child, ultimately murdering the infant despite Evy and Justin pleading with her not to. Evy then confesses that her mother has died, which she feels inadvertently responsible for by refusing to pray for her.

Evy rushes upstairs, her walls now covered in crayon drawings of Abyzou and dead, bloodied infants. She sees her mother standing in the bathroom, and is seemingly attacked while screaming for her to stop.