Leonora, an ageing prostitute, is on a crowded London bus when a waiflike young woman mutters the word "mummy" and follows her toward a cemetery. There, Leonora mourns the daughter she lost five years earlier at the age of ten, and she is struck by how much the girl resembles the child she once knew. This eerie sense of kinship leads Leonora to a bold decision: she agrees to accompany the girl to her opulent, lonely mansion and to become a surrogate mother to Cenci.
Cenci, a childlike figure who looks younger than her age, captivates Leonora with a mix of neediness and innocence. Over time, Leonora discovers that she bears a striking resemblance to Cenci's late mother, Margaret, and the two form a fragile, complicated bond as Leonora moves in to care for her. The dynamics inside the mansion begin to change as Leonora tries to fill the maternal role, even as the house is haunted by past tensions and unseen threats.
When the aunts Hannah and Hilda arrive uninvited at the mansion, they provoke Cenci and secretly steal valuables, a scene that Leonora witnesses with growing unease. Later, Leonora visits the antique shop where the aunts work, claiming to be Margaret's cousin, and learns more unsettling truths about Cenci’s history. The aunts reveal that Cenci is in fact 22 years old, despite her perpetual childlike appearance and behavior, and they recount how Margaret threw out Cenci's stepfather, Albert, after catching him trying to seduce her. Leonora, unsettled by these revelations, admonishes the aunts for failing to protect Cenci and warns them to stay away from the mansion.
Inside the mansion, a tense scene unfolds as Cenci is visited by Albert. After trimming his beard, he asks her to recreate the moaning sounds they used to make during their times together; she initially resists but eventually complies, and Albert kisses Cenci. Leonora returns to find a shaken Cenci cowering under a sink, mentioning Albert's visit. The two women soon check into a luxurious seaside hotel, a place of bright surfaces and hidden anxieties.
That night, Cenci arrives at the dining room sporting a fake baby bump, and Leonora pretends to accept the ruse, even as she struggles with the moral and emotional implications of the deception. The next morning, Albert confronts Leonora on the beach, accusing Cenci of being mentally unstable and claiming she has repeatedly tried to seduce him. Back in their hotel room, Cenci begins to kiss Leonora’s back during a massage, but Leonora pushes her away and insists on reality.
In a heated moment, Leonora tears off Cenci’s fake baby bump, sending the disturbed young woman into a frenzy. Later, from a balcony, Leonora witnesses Cenci and Albert in an intimate moment on the beach, a stark image that underscores the tangled, dangerous liaisons surrounding them. When Cenci returns to the room, she rejects Leonora and sends her away, severing the mother-daughter bond they had begun to build.
Back in London, Cenci slips into a dangerous darkness, overdosing on pills in a suicide attempt. Leonora later visits and begs to remain in the mansion, but Cenci rebuffs her again. As Leonora leaves, Cenci collapses and dies, a tragedy that hardens Leonora’s resolve. At Cenci’s funeral, Leonora stands beside Albert in silence, and in a moment of cold fury she stabs him to death. The film closes with Leonora lying on her bed, staring at a ceiling lamp as a children’s nursery rhyme of perseverance drifts through the air, a quiet, haunting echo of a life spent lingering between protective care and lethal consequences.
