History might be written by the winners, but nowadays the losers have a good shot at getting their versions out if they can entice the likes of Cate Blanchett and Robert Redford into embodying them onscreen. In Truth, Blanchett plays the former CBS News producer Mary Mapes, and Redford the anchorman — and, according to the film, Mapes’s surrogate father — Dan Rather. Based on Mapes’s memoir, Truth and Duty: The Press, the President, and the Privilege of Power, the movie is a righteously one-sided, unsubtle, and fairly effective account of “Rathergate,” the 2004 scandal that brought Mapes and Rather down: the discrediting of a letter that was supposed to prove that the young George W. Bush had not shown up for a good part of his Air National Guard service — itself the result of “Poppy” Bush using his influence to keep his son out of Vietnam.Did Mapes have any...