“Truth,” based on Mary Mapes’ 2005 memoir, “Truth and Duty: The Press, the President, and the Privilege of Power,” dramatizes her CBS reporting about President George W. Bush’s Texas Air National Guard service between 1968 and 1974.

CBS aired the story during the final months of the 2004 presidential election. Blowback from the story swiftly cast Mapes, CBS News and its senior newsman, Dan Rather, as sloppy journalists pursuing a political agenda.

Several things about Bush’s National Guard service aren’t disputed. He skipped a required physical; he stopped flying; he dropped out of training; he left the Guard early to attend Harvard Business School.

As seen in “Truth,” Mapes, Rather and their team were targeted not for the facts they reported, but for the authenticity of some documents in their report.

Writer-director James Vanderbilt, whose uneven filmography includes writing “Zodiac” and “White House Down,” makes his directorial debut with “Truth.” Given how potentially dramatic this story may have been, the results are unsatisfying.

Half of “Truth” is restlessness-inducing preamble. The other half shows Mapes, Rather and CBS News playing defense. But their defensive tactics feel like an argument that can’t be won.

However underachieving most of “Truth” is, Vanderbilt found a great actress to play Mapes. Cate Blanchett presents the producer, a behind-the-scenes star at CBS New, as a tough journalist who grabs a stories by the tail. Mapes’ award-winning reporting includes CBS’ explosive story about American military personnel torturing prisoners at Abu Ghraib prison in Iraq.

Of course, “Truth” is Mapes’ story, told from her perspective. Like Rather, she was an experienced broadcast journalist. Her immediate triumph before the Bush-National Guard debacle is the Abu Ghraib reporting, which won a Peabody Award.

Blanchett rides an emotional, professional roller coaster as Mapes. In just hours, the dedicated journalist slips from star to perceived hack. Blanchett passionately throws herself into the role of journalist under fire, but the script lets her down.

Blanchett’s co-star, Robert Redford, in the role of veteran anchor Rather, is essentially a supporting player. A largely low-key presence in the story, Rather never raises his voice. He usually keeps a cool head above the fracas.

The good-natured aloofness Redford applies to the character is a fitting approximation of the real Rather, whose reporting for CBS and 24 years as the network’s evening news anchor demonstrated that same steely serenity.

Further down the cast, Dennis Quaid, Elisabeth Moss and Topher Grace portray Mapes’ investigative team. The actors are underused. Their characters’ digging doesn’t translate well on screen. Grace’s big speech amounts to noise and fury signifying nothing.

Despite Blanchett’s and Redford’s earnest work, the truth about “Truth” is that it fails to engage on the emotional and intellectual levels that would have made it commanding drama.