Jackie Kimball is a normal 17-year-old girl – except that she has been in love all her life with Matthew, her 18-year-old brother. Matthew, who does not share Jackie’s incestuous inclinations, has just acquired his first girlfriend, and Jackie is in crisis, unable to eat or sleep and disrupting family life. The siblings remain close, despite Jackie’s dramatic expressions of her misery.
Jackie's widowed mother Alice is a sympathetic but detached presence, often found at her writing desk. The family includes an older brother, studying abroad, and a sister who is impatient with Jackie's flamboyance. Everyone prefers to interpret the attraction as a long-standing phase.
Matthew's unexpected breakup provides Jackie temporary relief, and the siblings pass a pleasant Brooklyn summer together. But Matthew's departure for college hits Jackie hard, and her mother sends her to psychotherapy. As Jackie becomes close to her therapist, she and Matthew keep up contact via electronic media.
A restless Jackie tries her hand with boys and seduces an attractive classmate, whom she alienates without a second thought when Matthew returns on vacation. Regaling her brother with her sexual explorations, the emboldened Jackie lets her usual suggestiveness turn into open seduction. Matthew finally reaches his breaking point and nearly severs relations with his sister but the siblings make a sad peace that feels like a farewell.
Facing the task of moving forward, and her own impending departure from home, Jackie acquires a boyfriend and a career interest in psychology. Perhaps living without what you want most isn't the end of the world…