A thrilling whitewater rafting trip takes a dark turn as three long-standing friends embark on a journey into the unknown. Leading the expedition is the fearless adventurer, Juno, portrayed by Natalie Mendoza. Joining her are Sarah, played by Shauna Macdonald, who is both a wife and mother, and her closest friend Beth, portrayed by Alex Reid. Awaiting them at the finish line are Sarah's husband, Paul (Oliver Milburn), and their daughter, Jessica (Molly Kayll). An unsettling moment occurs when Paul and Juno share a brief exchange before he departs with Sarah and Jessica. Unfortunately, their journey takes a tragic turn when Paul, distracted during a discussion of his absent-mindedness, crashes their car head-on into a truck carrying metal rods, which results in one of those rods piercing through his skull.
Devastated, Sarah begins to experience haunting dreams where Jessica appears to offer her a birthday cake. After waking up in the hospital, still traumatized from the accident, Sarah has a terrifying episode where she feels the hospital walls closing in around her. It’s only through the comforting embrace of Beth that she realizes the heartbreaking truth: Jessica is gone. A year passes, and in an effort to help Sarah escape her isolation, Juno organizes a spelunking expedition in the Appalachian Mountains. The group gathers in a cabin before the adventure, joined by additional companions, including Juno's adventurous friend Holly, played by Nora-Jane Noone, and two European sisters, Rebecca, played by Saskia Mulder, and Sam, portrayed by MyAnna Buring.
However, tensions simmer as both Juno and Beth harbor unresolved feelings about the past, particularly Juno's departure the day of the accident. That evening, as drinks flow and laughter echoes, Sarah clings to her husband's motto "Love Each Day" to motivate her participation in the impending adventure. In her sleep, she is haunted by nightmares of metal rods, reminiscent of the terrible day her family was shattered. The next morning, Juno leads the group on a challenging drive, seemingly neglecting to bring the cave map necessary for navigation. Upon arrival at the cave entrance, they are confronted by a foreboding sight: a gutted bull elk carcass.
Despite initial reluctance, the women rappel into the abyss, only to find darkness engulfing them. Juno ignites flares to illuminate the colossal cavern before them, even catching Holly's fascination. An emotional moment arises when Juno apologizes to Sarah for her past actions, sparking Sarah’s decision to explore a narrower tunnel. Unfortunately, a rock dislodges, sending Sarah into a panic. In a rush to help, Beth reassures her that the worst has already happened, but when they both escape just in time, the return route crumbles behind them.
In a shocking revelation, Juno admits to deliberately leading them into an unexplored cave rather than the one they were supposed to traverse. Fueled by ambition to discover new territory, Juno's intentions create discord within the group, particularly with Beth, who reminds Juno of her abandonment when Sarah needed her most. In an effort to survive, they navigate a precarious precipice but lose their climbing gear in the collapse. Rebecca undertakes a dangerous climb to secure a new rope system, only to find herself injured when attempting to help Juno cross.
As they journey deeper, dark shadows loom. The tension escalates when a dark creature reveals itself to Sarah alone, whose warnings fall on deaf ears with the group. Realizing too late that they have trespassed into a sinister domain, Holly tumbles into a pit, leaving the group scrambling to provide aid. The nightmare intensifies as creatures of unknown origin begin their attack, each confrontation leading to shocking fatalities within the group. The horror escalates when Sarah is forced to confront the gruesome ends of her companions, blurring lines of trust and survival.
In the ultimate climactic moments, betrayal simmers as Juno and Sarah find themselves at odds amid a struggle for life amongst the malign forces of the cave. With every attempt to escape, they are met with insurmountable odds as distress and bloodshed unravel their fragile bond. In a heartbreaking finale, when all seems lost, Sarah's flight to freedom reveals itself as merely another layer of madness, stripping her of reality. She awakens to the bone-laden darkness, where creatures await, and desperately clings to fading memories of her daughter amidst the chaos that surrounds her.
