Twisted Nerve (1968)

In London, twenty-two-year-old Martin Durnley (Hywel Bennett) lives with his well-off mother and stepfather, Enid (Phyllis Calvert) and Henry Durnley (Frank Finlay), the latter who earns a living as a banker. Martin is the only connection his institutionalized older brother Pete has to the family, Pete who has Down Syndrome, and who is not expected to live much longer out of natural course.

Enid has long coddled Martin in her gratitude that he did not have Down Syndrome, while Martin and Henry do not get along in Henry's belief that Martin has a sense of entitlement. Martin is mentally disturbed, as demonstrated when he meets by chance Susan Harper (Hayley Mills), who is going to school to become a teacher and who has come home temporarily working at a library while she continues her studies. Her divorced mother, Joan Harper (Billie Whitelaw), takes in boarders both as income and for company (in many aspects of the word) especially while Susan is away. Her two current boarders are crass Gerry Henderson (Barry Foster), with whom Joan does occasionally share her bed, and Indian national Shashie Kumar (Salmaan Peerzada), who works at a nearby teaching hospital as an intern. Martin presents himself to Susan as mentally slow "Georgie Clifford" - always referring to himself in the third person - who Susan takes under her wing as "Georgie", enacting an elaborate plan, does whatever he can to insinuate himself into Susan's life in his romantic infatuation with her. In Martin's psychopathy, he has the potential to harm anyone who frustrates him which thus places the lives of Susan and those around her in danger if they do anything against "Georgie".