A body is discovered in the attic of a tenement house on Coulthard Street, sparking a meticulous yet unsettled investigation. The remains belong to Melissa Young, a young woman who owned the top flat and whose death went unnoticed for two years. Detective Len Harper, already edging toward retirement, continues the inquiry with a quiet resolve. Disturbed by the idea that Melissa’s death could be overlooked, he keeps the case alive as a way to stave off his own isolation, even as his colleagues close it as a suicide.
The residents of the building appear outwardly content, each living their quiet dramas beneath the same roof. A young couple, Michael Jenson and Vidya Khan, have just moved in and are expecting a child; designers Elaine Markham and Peggy Scott share a space and a life; journalists Kieron Moss and his partner Patricia Keenan discuss a future together; and middle-aged maths teacher Joe Sellers shares a flat with the much younger Liz Fletcher, whose presence is domestic yet tantalizingly ambiguous. Len’s search through Melissa’s flat stirs trouble when Liz attacks him and escapes, leaving his memory of the face as a blurred challenge.
As Len digs, he learns Melissa had been used in a lingerie photo shoot by Elaine and Peggy, a fact Melissa had attempted to distance herself from through legal counsel, which left Elaine’s threats hanging in the air. Vidya, trying to understand Melissa’s life, creates a social media footprint for her and tracks down a friend who had joined Melissa in a slimming group. That friend paints a picture of Melissa that clashes with how the neighbors remember her and reveals that Melissa had a boyfriend—someone later identified as Kieron, who is navigating a tense rebound between his divorce and Patricia.
Kieron’s world bleeds into the apartment’s other relationships when his son Adam Moss meets his father’s girlfriend Patricia and responds with unsettling behavior. Meanwhile, Kieron’s own vulnerabilities come to light when, after a night of drinking, he tries to force himself on Patricia. She resists and moves out. Peggy’s own fragility surfaces as she attempts to self-harm, only to be dismissed by Elaine, who responds with a brutal episode of control and confinement. The tension in the building grows as Michael begins an affair with Liz, who is living in Joe’s flat, a development that further strains the uneasy alliances among the tenants.
Joe, aware of the burgeoning secrets, eventually confronts Michael about the affair. His anger spills over into an impulsive act against a pupil, and in a moment of crisis, he spontaneously confesses to Melissa’s murder while in police custody. Len, convinced that Joe resented Melissa for someone else’s harm in her past and believing that to be the motive, pushes the idea that Joe is the killer. The police, however, see Joe’s confession as a veiled attempt to shield another. In truth, Joe’s confession is a misdirection, a pretense to shield Liz, who has a long and troubled history of violence—one that includes the killing of her abusive stepfather years earlier, in which Joe helped her evade detection by offering her a safe place to hide.
As the web tightens, Len discovers that Liz’s behavior in the building is erratic and dangerous. He persuades his former colleague Alice Yapp to look more closely at Liz, and together they form a plan to confront the danger when Vidya finds herself alone with Liz in the house. The tension erupts as Liz reveals her affair with Michael and attacks Vidya. Len and Alice arrive just in time to arrest Liz, preventing harm to Vidya and her unborn child. Michael’s defense—that his affair was simply an act of revenge against Joe—fails to reassure Vidya, who recognizes that Michael’s fixation on revenge shows a lack of readiness for parenthood and a stable relationship, and she decides to part ways with him.
Months pass, and life in the building appears to inch toward resolution. Liz is in custody, Kieron reconciles with Patricia, and Len’s friendship with Vidya deepens as he moves into Melissa’s former flat to act in loco parentis to Vidya and her child. Yet a chilling discovery interrupts this fragile calm: Peggy has died under unclear circumstances, and Elaine—now utterly unhinged—has hidden Peggy’s body in the bath, refusing to let her lover leave. Len confronts Elaine, only to be stabbed as the confrontation spirals.
Vidya returns to the scene, finds Len badly injured, and hides with her baby as Elaine closes in. The attic becomes the stage for a deadly game, and as the two women maneuver in the darkness, Len’s earlier suspicions about Liz resurface in a new light. Yet the true killer is not Liz; flashbacks reveal that Peggy was Melissa’s killer. Melissa had confided to Elaine that Peggy planned to move out, and when Peggy could not admit this truth, she killed Melissa. Elaine, leveraging Peggy’s guilt to keep her bound to the life she had built, left Melissa’s body in the attic to serve her own purposes.
In a tense finale, Vidya uses the attic’s precarious vantage to outmaneuver Elaine, pushing her through a trapdoor and sending her to the floor below near Len’s flat. Elaine’s resolve weakens as Len, who had reached his own flat, shoots her with a bow. Len and Elaine lie injured, and Vidya, though unhurt, is distraught. The arriving police are met by Vidya, who ascends to safety while Len, gravely wounded and facing the possibility of death, asks that she stay with him, revealing a final, intimate plea: he does not want to be alone.
