When Walt Disney's daughters begged him to make a movie of their favorite book, P.L. Travers' "Mary Poppins," he made them a promise-one that he didn't realize would take 20 years to keep. In his quest to obtain the rights, Walt comes up against a curmudgeonly, uncompromising writer who has absolutely no intention of letting her beloved magical nanny get mauled by the Hollywood machine. But, as the books stop selling and money grows short, Travers...