Evangeline "Evy" Babic is a grounded skeptic who runs The Undertone, an online horror podcast, alongside her believer co-host Justin. Evy has recently moved back into her childhood home to provide round-the-clock palliative hospice care for her comatose, dying mother. While navigating personal turmoil and an unexpected six-week pregnancy she feels unfit to carry, Evy and Justin receive an anonymous email with ten strange audio recordings.

The audio files document a young couple, Mike and Jessa. Mike set up microphones to capture Jessa's bizarre sleep-talking episodes, which begin as incoherent muttering but escalate into disturbing backward singing of "London Bridge". As Evy scrubs through the audio, reversing and filtering the clips, she discovers hidden demonic phrases and links to the Mediterranean demon Abyzou, an entity of folklore infamous for targeting pregnant women and newborns due to its own infertility.

Paranoia takes hold as the physical reality in the house starts echoing the audio clips. Religious icons shift locations, the comatose mother moves unnaturally in her bed, and live phone callers to the podcast reveal that Mike and Jessa were found dead under gruesome circumstances. When Evy confesses her guilt over stopping prayers for her dying mother, the boundary collapses, sealing her fate inside the darkness.

In the frantic third act, live callers begin ringing the podcast line. A neighbor reveals that Mike and Jessa were discovered dead with bags over their heads in a room covered in baby sketches, with Jessa having been pregnant. Another caller demands to speak to "Mary", the exact name Evy privately wished to give her unborn child, before a final caller brutally murders her own crying infant live on the call while begging for mercy.

Evy breaks down and confesses that her mother has passed away. She carries crushing guilt, knowing she refused her mother's dying requests to pray together. Rushing through the dark home, Evy discovers the walls smeared with frantic crayon illustrations of Abyzou and infant gore. The house in the recordings was her own all along. Entering the bathroom, she is confronted by the standing, reanimated form of her mother. Overcome by supernatural paralysis and despair, Evy whispers "come in, Abyzou," fully surrendering herself to the demon.