Twisted Nerve [1968 - UK] thriller

In London, 22-year-old Martin Durnley (Hywel Bennett) lives with his well-off mother Enid (Phyllis Calvert) and stepfather Henry Durnley (Frank Finlay), a banker. Martin is the only connection his institutionalized older brother Pete, who has Down Syndrome, has to the family. Enid has long coddled Martin in her gratitude that he did not have Down Syndrome, while Martin and Henry do not get along; Henry believes that Martin has a too-high sense of entitlement. Martin is mentally disturbed, as demonstrated when he happens to meet Susan Harper (Hayley Mills), who is going to school to become a teacher and has come home temporarily, working at a library while she continues her studies. Her divorced mother Joan (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 occasionally shares her bed, and Indian national Shashi 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--and as Susan takes "Georgie" under her wing, Martin enacts an elaborate plan to do everything he can to insinuate himself into Susan's life as his romantic infatuation for her grows. In Martin's psychopathy, he has the potential to harm anyone who frustrates him, which places the lives of Susan and those around her in danger if they do anything against "Georgie".