Dahlia, portrayed by Jennifer Connelly, relocates with her daughter Ceci -also known as Cecelia- played by Ariel Gade to a new apartment in a condominium on Roosevelt Island, New York, due to her impending divorce from Kyle (played by Dougray Scott). The transition struggles are amplified by Kyle's refusal to support Ceci's new school and environment, leading to a tense custody battle. The mediators involved, Bill Buell and Linda Emond, express concerns that Ceci is the one who might suffer the most regardless of who gains custody.
Settled on the ninth floor, Dahlia attempts to maintain a normal routine, reading children’s stories to Ceci and dropping her off at school. Yet, she is plagued by excruciating headaches that necessitate strong medication. The building has a dismal atmosphere, with a grumpy clerk named Veeck, played by Pete Postlethwaite, who constantly complains but does little to help.
Overcome by exhaustion and the pressures of legal battles, Dahlia finds herself in a crumbling state of mental health, especially with a controlling mother, played by Elina Löwensohn, adding to her stress. Upon moving in, she notices a troubling damp stain in the bedroom that becomes a significant issue as they hear odd sounds throughout the apartment, exacerbated by the poor lighting and ominous hallways. Although Veeck promises to send a plumber, nothing improves.
As Ceci struggles to make friends, she turns to an imaginary companion named Natasha. Despite Dahlia's efforts to remain steadfast, the burden of her situation weighs heavily on her. One day, after a troubling laundromat episode where Ceci believes she sees a girl inside the washing machine, Dahlia dismisses it as a prank from a couple of troublemaking boys. However, the strange occurrences increase when Ceci is driven into fits of anxiety at school, sparking concern from her teacher, played by Camryn Manheim.
Following a series of unsettling events, including Ceci fainting in the school bathroom, Kyle hurls accusations at Dahlia's capability to parent. In a bid to help, their lawyer, Jeff Platzer (played by Tim Roth), intensifies his support by calling the building owner, John C. Reilly, who is alarmed by the water damage emanating from the upper floor.
A journey of discovery leads Dahlia to learn about the tragic history of the apartment’s 10th floor, once inhabited by a troubled Russian family and a little girl named Natasha, who ultimately met a tragic fate. Realization dawns when Dahlia finds Natasha’s corpse in a neglected reservoir, only to escape as Natasha seemingly reaches out to pull her in. In the wake of this horror, when it seems the turmoil is at an end, Dahlia strives to rewrite her relationship with Kyle for Ceci's sake.
However, as she attempts to create a bath for Ceci, a chilling transformation occurs when she discovers she is with the ghost of Natasha instead of her daughter. In a desperate act to save Ceci from drowning, Dahlia proposes to care for Natasha, leading to a harrowing flood in their bathroom.
In a harrowing finale, police arrive assuming Dahlia might have attempted suicide, while in an emotional closure, she reassures Ceci from beyond, promising to always be there if she’s ever in need. The heavy weight of the events unfolds, leaving Dahlia to linger in the building, bound together with the spirit of Natasha.
