Alison Patricia Neal appears to lead a contented life as a wife and mother to two daughters, despite the fact that she is blind. Her blindness resulted from a fall during pregnancy, but she is supported by her devoted husband, Eric Crawford, a reputable London businessman, along with her elderly French maid and a close family friend named Paul. Alison's sense of normalcy is challenged when her younger sister, Robin Samantha Eggar, returns to their family home, bringing with her an air of flirtation and trouble.
Robin's visit sparks tension and unease. She takes Alison shopping and strongly urges her to see a specialist about her eyesight, implying that there might be a medical solution. Alison, however, believes her blindness is neurological, saying that a doctor told her her mind refuses to process what it has seen—an assertion that hints at deeper psychological issues rather than purely physical ones. The story takes a darker turn when Robin's behavior becomes increasingly provocative and unpredictable. She teases Alison and Eric in ways that suggest she might be seeking to manipulate or punish them. Her flirtations often border on provocative, and at one point, Robin jokes about seduction and revenge in front of Alison and Eric.
A pivotal scene unfolds in the family’s town house, where Robin's erratic behavior escalates. She becomes aggressive and even physically attacks Eric with sewing scissors after he reacts violently to her teasing. Alison, unable to see the altercation, senses the shift in atmosphere and is shocked when she asks about Robin’s attire, feeling her sister's skimpy nightdress. Later, Alison reflects on her mysterious memory of the night she lost her sight, recalling waking up in labor without her husband by her side and hearing Robin crying. She remembers leaving her bedroom, the fall she took, and waking up in a hospital, but the exact sequence of events remains hazy.
As the family visits their country house, more revelations emerge about their past and Robin’s complex relationship with them. Their mother hints that as children, Alison often took Robin’s belongings and suggests that Robin was perhaps infatuated with Eric long before their marriage. Alison retorts that it was just an adolescent crush, implying Robin’s early interest may have been more profound than she admits.
In town, a conversation over dinner with Paul reveals Robin's flirtations and her history—she has been married and left her husband in the U.S., and her mother suspects she now seeks escapades with younger men, evident in her flirtations on the beach. Robin admits bitterly to Alison that she hated Eric, claiming that her feelings stemmed from him taking Alison away from her. She confesses her pain about her lost innocence, recalling a tragic night on the beach when she was a girl—an event she'd wanted to share with Alison but felt unable to.
The tension heightens when Robin’s actions at the country house cause a scene, including stealing black currants with the children. Later, she shares a tender moment with Paul, and he asks why she refuses to marry him. Robin confesses that she worships Eric, believing he gave her only a fragment of love, and that her intense longing for him was both admiration and desire—though she admits she is troubled by her feelings. The emotional complexity and unresolved tensions between the characters surface repeatedly, highlighting Robin’s instability and her destructive influence.
As the story progresses, Alison seeks honest advice from Paul about his relationship with Robin. She warns him that love can be "like committing suicide" if he isn’t careful. Meanwhile, Robin continues to sow discord, suggesting to Alison that Eric views women as either "bitches or objects of pity," and comments cruelly about Eric's infatuation with her blindness as a way to keep her obedient.
The climax of the film occurs when Robin tries to leave on horseback, leading to chaos in the household. Alison is struck by the horse and falls, but she reassures Eric from the grass that she is alright. Robin, now unconscious, is carried into the house, and a series of tense moments lead to Alison witnessing her sister and Eric, in a flashback, in a deeply intimate and forbidden scene. The memory is powerfully emotional, revealing Robin’s obsession and the complicated bond they share.
Robin, regaining consciousness, confronts Eric with defiance, refusing to accept being second best, and suggests he leave Alison and the children. Eric’s fury escalates, and he begins to strangulate Robin in a violent display. Robin, however, laughs at him and then turns away, crying, demonstrating her disturbed state and the fragile line between love and destruction.
Alison, meanwhile, is caught in her own turmoil, quietly observing her family’s disintegration. She sees her children playing outside, but her heart is heavy with sorrow. An attempt by Robin to seduce Eric ends unsuccessfully, and Alison, overwhelmed with emotion, prepares for an ending. She arranges phenobarbitone tablets into an "A," symbolizing her perhaps planning to surrender to her despair. Her family continues to go about their day, oblivious to her internal struggle.
On the day of what appears to be an emotional anniversary, Robin arrives with roses, falsely celebrating. She boldly announces her intention to marry Paul, and her behavior further deteriorates, her actions revealing deep-seated emotional wounds. As she confesses her love and desire for Eric, her words and gestures betray her inner pain. Eric, overwhelmed, is seen sobbing as he embraces her, while Paul, dismayed, leaves the scene.
The final moments capture Alison, quietly walking into the garden, gazing skyward with a faint smile amid her heartbreak. The story concludes on a note of poignant ambiguity, highlighting the tragic unraveling of family bonds and the complex, often painful, nature of hidden psychological scars.
