“Plimpton! Starring George Plimpton as Himself,” which will have its nationwide broadcast premiere tomorrow at 9 p.m. on PBS and locally on WNET, is a spellbinding film that chronicles the life of a singular man. George Plimpton grew up in a duplex apartment on upper Fifth Avenue, attended St. Bernard’s School, Phillips Exeter Academy, Harvard -— where he was a classmate and close friend of Robert F. Kennedy — and Cambridge University. But even if you know nothing about his background, you have only to hear him speak to know he was a patrician.