Tricia, Courtney Bell, a pregnant woman who has lived alone in a Glendale neighborhood for seven years, begins to confront the possibility that her missing husband Daniel, Morgan Peter Brown, may never return. As she contemplates declaring him dead in absentia, her younger sister Callie, Katie Parker, a former addict, arrives with a copy of The Three Billy Goats Gruff, a children's book whose simple fairy-tale logic quietly shadows the film and hints at a bridge guarded by an unseen, malevolent force. The two women team up to secure Daniel’s death certificate and help Tricia find a new place to live, all while the more immediate task of surviving their increasingly tense days in their aging neighborhood weighs on them.
Callie’s presence injects a restless energy into Tricia’s careful plans. As the date to finalize Daniel’s status approaches, Tricia also begins seeing unsettling nightmares and experiences nerve-wracking anxiety, which her psychiatrist, Dr. Elliott, Scott Graham interprets as the natural stress and guilt of an unresolved loss. The tension between keeping a fragile normalcy and facing a possible truth pushes the two sisters toward a decision about their lives and their future in the same small space they’ve long inhabited.
During a jog, Callie encounters a gaunt man in a tunnel who seems simultaneously aware of her and shocked that she can see him. He introduces himself simply as Walter Lambert, and he asks her to contact his son. The encounter leaves her unsettled, and when she returns later with food, she finds the tunnel seemingly empty, a mystery that lingers in the back of her mind. Soon after, Callie notices a pile of small metal objects on their doorstep and decides to move them toward the tunnel entrance, even as a passerby warns her to stop. The next morning, the objects reappear on her bed, adding to the sense that something else is at work in their neighborhood.
Detective Mallory, Dave Levine, is meanwhile pursuing Daniel’s disappearance and is emotionally entangled with Tricia, a dynamic that complicates both professional and personal lines. Callie relapses into drug use in the midst of the mounting tension, and Tricia resolves to sign Daniel’s death certificate while also trying to maintain some sense of life beyond the investigation—even planning a date with Mallory. But the night takes a sudden, harrowing turn when a bloody, barefoot Daniel appears at Tricia’s door, his body telling a story of hardship and fear she cannot ignore.
In the hospital, Daniel is diagnosed as severely malnourished, dehydrated, and physically abused. He seems stunned that others can see him and can only mutter that he was “underneath,” as if the tunnel before his home holds secrets he cannot fully explain. He can communicate little, and his fear is directed at the tunnel that seems to loom in the front of their residence. Mallory tries to persuade Tricia to leave Daniel behind and pursue a future with him, but Tricia refuses the obvious separation and the two even share a kiss in a moment of conflicted longing.
Daniel reveals a startling detail: he made a bargain with an insectoid creature hidden within the walls of the tunnel. Callie, meanwhile, investigates strange movements in a bathroom and is knocked unconscious. When she comes to, she witnesses the creature pulling Daniel into the tunnel and through the tunnel’s solid walls, a spectacle that challenges all of the sisters’ understanding of reality. Faced with these inexplicable events, Callie hides the relapse, and Tricia discovers her sister’s stash, accusing Callie of hallucinating everything.
Callie then uncovers troubling online information about local disappearances spanning a century, including references to Walter, and a local resident who claims his son was indeed taken by monsters. The next day, Walter Lambert’s body is found at the tunnel’s entrance, and the revelation that the man Callie met was his son, Jamie, shifts the investigation’s trajectory. Jamie implies that he has been offering pets as sacrifices in a desperate attempt to bring his father back, a claim that intensifies the fear weaving through the investigation. The sisters are attacked again by the creature, who abducts Tricia.
In a desperate bid to rescue her sister, Callie files a missing person report and proposes a dangerous trade—her own life in exchange for Tricia’s safety. The creature, however, returns not Tricia’s body but Tricia’s unborn fetus, leaving only her empty shoe behind as a chilling symbol of what has been lost. Mallory, meanwhile, uncovers Callie’s envelope of research and shows up at the edge of the tunnel with questions. Lonergan, a fellow detective Justin Gordon, remains skeptical about the wider implications and jokes about indefinite case coverage, while Mallory stubbornly holds onto a stubborn belief that Callie may have escaped with Tricia somewhere safe.
The film closes on a bleak, eerie note as Mallory places posters around town while Callie appears as a spectral figure in the tunnel, an apparition that seems at once present and unreal. When Mallory walks away, Callie’s presence remains—occluded by the creature’s looming influence—as if the mystery has only just begun and the line between reality and the tunnel’s otherworldly reach has blurred beyond repair. The ending leaves viewers with a haunting sense that the bridge between two worlds, hinted at by a fairy-tale tale, remains guarded by something unseen, waiting to claim those who dare to listen to whispers from the tunnel.
