Film archivist David has been facing significant turmoil lately, fueled by the unsettling suspicion that his wife Alice may be having an affair with Alex, a client from her workplace. The pressure escalates when his work partner Claire hands him a reel of footage for archiving, which reveals that their home was the site of a horrific murder back in 1902. The tension reaches a peak when David covertly follows Alice after work, only to witness her in an intimate encounter with Alex, shattering his heart. In a fit of rage, he seizes a hammer but ultimately regains composure and discards it into a nearby canal.
Feeling nauseated, David rushes to a public restroom that his young son Billy believes to be haunted. There, he becomes ill and hears a chilling voice murmuring incomprehensibly. Before collapsing, David glimpses what he believes to be a spectral figure committing an act of violence against Alice. As Alice fails to return home, he contacts the police, and upon being questioned by Detective McNamara, David denies any knowledge of the affair and omits the eerie incident at the canal. Later, the authorities recover Alice’s body submerged in the canal but conclude her death to be accidental.
As David mourns and heals, he spirals into an obsession with a series of unresolved murders dating back a century that haunt the canal and surrounding area. This fixation troubles Claire and strains his relationship with Sophie, the nanny for Billy. David's actions become erratic as he borrows early 20th-century cameras to document the surroundings, hoping to capture evidence of any supernatural phenomena. Convinced that he has sighted ghosts, he relocates Sophie and Billy to a hotel for their safety.
During a video chat with Billy, David observes a ghostly silhouette lurking in the background, which prompts him to break into the hotel room, scaring Sophie in the process. When David refuses to discuss his increasingly strange conduct, Sophie informs him of her intention to resign the following day. That night, circumstances take a dark turn as Sophie is assaulted. In a desperate attempt, David locks her in a closet with Billy while he searches their home, now isolated.
Desperate for clarity, David turns to Claire for support in developing the footage he has captured; despite her hesitance, she ultimately agrees. When police uncover the hammer from the canal, suspicions arise regarding David's involvement in Alice's death. His lawyer manages to negotiate a deal ensuring Billy remains with him pending a psychiatric evaluation, although David adamantly refuses to comply.
Claire visits David to deliver the developed film, and he persuades her to view it alongside him. Initially dismissive, she is soon terrified by a ghostly figure creeping closer, emerging from a hole in the wall and subsequently attacking her. In disarray, David rushes out, only to collide with McNamara, who has been surveilling the premises. A frantic chase ensues as police pursue David and Billy, leading them into a concealed tunnel leading to the canal.
David begins recalling the tunnel’s history, which he believes was used for dark rituals by devil-worshippers involving child sacrifices. Here, he is confronted by ghostly visions of Alice, the voice from the restroom urging him towards violence. Wracked with fury, he recalls drowning her in the canal, and when Claire failed to see anything on the footage, he ended her life as well.
Upon reaching the canal, a ghostly entity pulls David beneath the surface, and although he does not resist, he aids McNamara in rescuing Billy. In the aftermath of David’s tragic demise, Billy's grandmother decides to sell the house and takes custody of him. Just as Billy rushes back inside the house to grab a toy, David’s spirit materializes from behind the wall, beckoning him to join him forever in the house if he follows his instructions. In an impulsive act, Billy leaps from his grandmother’s car as they drive away. Meanwhile, back at the house, the real estate agent watches with satisfaction as she sees Billy ascend the stairs and slam a door, hinting at the lingering presence of David's spirit.
