Family Plot (1976)

A phony psychic, Blanche, and her actor-turned-cab-driver boyfriend, George, are hired by wealthy Julia to locate her long-lost nephew, given up for adoption years earlier. Julia, haunted by guilt, offers $10,000 to uncover his whereabouts. As the couple digs deeper, they uncover a dark truth: the nephew, now known as jeweler Arthur Adamson, faked his death and is involved in a series of kidnappings, exchanging hostages for diamonds.