In its first hour, “Truth” feels like an academic lecture. Hour one — during which “60 Minutes” producer Mary Mapes and her team are on the investigative hunt — should be exciting, but it’s dull.

“Truth” finds momentum in its second 60 minutes. That’s when Mapes and her “60 Minutes” colleagues, including veteran CBS newsman Dan Rather, become the investigated.

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

CBS aired the story during the closing-months of the 2004 presidential election. Blow-back from the report swiftly cast Mapes, CBS News and senior newsman Rather as sloppy journalists who pursued 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; and he left National 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 “Zodiac” and “White House Down,” makes his directorial debut with “Truth.” Given how dramatic the story might have been on film, the results are unsatisfying.

Half of “Truth” is restlessness-inducing preamble. The other half shows Mapes, Rather and CBS News playing defense. Their defensive tactics feels 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 News, as a tough journalist who grabs a stories by the tail. Mapes’ honored reporting includes CBS’ explosive story about American military personnel torturing prisoners in Iraq’s Abu Ghraib prison.

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

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

Blanchett’s co-star, Robert Redford in the role of Rather, is but a supporting player. Rather is a largely low-key presence in the story, never raising his voice, usually keeping 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 and their characters’ digging doesn’t translate well on screen. And 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.