Tess Marshall arrives at a rental property in the dilapidated neighborhood of Brightmoor in Detroit the night before her job interview. To her surprise, she discovers that the house has been double-booked and is currently occupied by a man named Keith Toshko. Although initially unsettled by Keith, Tess gradually lets her guard down and decides to spend the night while he takes the sofa. In the middle of the night, Tess wakes up startled to see her bedroom door ajar; she questions Keith, who insists he didn’t interfere with it.
The following morning, Tess goes for her interview and is warned to remain vigilant in the unsafe neighborhood. Upon returning to the rental, she finds herself pursued by a homeless man who shouts for her to leave. In her frantic escape, she accidentally locks herself in the basement and discovers a concealed corridor that leads to a disturbing room filled with a camcorder, a stained mattress, a bucket, and a bloody handprint.
When Keith returns and frees Tess, he investigates the hidden passage. However, when he fails to reappear, Tess ventures down the corridor and uncovers a tunnel where she finds Keith injured and disoriented. Heightened tensions escalate as the duo is attacked by a grotesque figure known as "The Mother," who brutally kills Keith.
Two weeks later, the house owner, AJ Gilbride, is let go from his television role amid sexual assault accusations. Struggling to pay legal fees, AJ heads to Detroit to assess the property for sale. While inspecting the house, he discovers the hidden tunnel and unwittingly encounters The Mother, who attacks him. He falls into a hole where he unexpectedly reunites with Tess.
A haunting flashback reveals the house's sordid past, detailing how Frank, the original owner, abducted women and imprisoned them in the tunnels for horrific purposes, including the systematic abuse of their offspring.
In present times, The Mother captures Tess and AJ, revealing her twisted desire to have them as her children. As AJ is forcibly breastfed, Tess manages to escape with the aid of Andre, the initial homeless man who warned her about the dangers lurking in the neighborhood.
With newfound courage, Tess leads the police back to the house, but they dismiss her claims. Undeterred, she retrieves her car keys and recklessly rams The Mother with her vehicle, seemingly putting an end to the threat. However, chaos ensues when AJ mistakenly shoots Tess, believing she is The Mother. The two manage to escape, only to find themselves in a gripping confrontation where they discover The Mother has inexplicably vanished.
In their final standoff, the truth of The Mother's heritage as a byproduct of multi-generational incest emerges through Andre's distressing explanation. In a harrowing turn of events, The Mother reappears, violently killing Andre before pursuing Tess and AJ. As they seek refuge in a water tower, AJ makes a treacherous decision to save himself, pushing Tess off the structure.
In an unexpected twist, The Mother sacrifices herself to cushion Tess's fall, but AJ's attempts to justify his selfishness lead to his demise when The Mother exacts her wrath. Ultimately, Tess finds the strength to confront The Mother, firing the gun that once belonged to Frank, and leaves the scene as dawn breaks, a somber reminder of the horrors she has faced.
