CAMBRIDGE, Mass. — The most damning part of a new documentary about Beatrice Mtetwa, a lawyer who’s spent decades fighting human rights abuses in Zimbabwe, comes from the mouth of President Robert Mugabe’s former minister of information.

“All countries are ruled by men and women,” Jonathan Moyo tells filmmaker Lorie Conway, “and the law becomes what they say it is.” Mtetwa agrees, in a way: “Unlike a lot of other dictators, Robert Mugabe doesn’t just go out and do what he wants,” she says in the film, ‘Beatrice Mtetwa and the Rule of Law.’ “He first goes to parliament and passes a law and says it’s now legal to punch somebody in the nose.”

For the same three decades that Mugabe has been doing that with such impunity, Mtetwa has been trying to use the constitution to defend people such as peace activist Jestina Mukoko, who was detained and tortured for 89 days in ’08. Or Andrew Meldrum, the last resident foreign correspondent in Zimbabwe, who was arrested for practicing journalism, essentially, then thrown in a car with a hood over his head and deported straight from the courtroom where he’d been found not guilty of any crime.

Again, it’s Moyo, a member of Parliament from Mugabe’s ruling party, who confirms that there’s no real freedom of speech in Zimbabwe when he explains the expulsion of Meldrum, who wrote for the Economist and the Guardian during his 23 years in the country, and is now with the AP in South Africa:

“If journalists and especially their governments become hostile to our country and declare that they will use the media to pursue that hostility toward our country, then we have a right to stop those people,” he says in the movie. “They can write whatever nonsense they want to write outside our borders.”

The film had to be made in secret, and will be shown that way, too, inside Zimbabwe. And Mtetwa herself was arrested last month and charged with obstruction of justice when she asked police searching a client’s home to produce a warrant. Her trial, originally scheduled for May, has been delayed.

On Tuesday, she was scheduled to fly to Washington, D.C., for a Thursday showing of the film at the U.S. Institute for Peace. When I spoke to her on the phone as she was packing, she said she was not too afraid during this most recent incarceration:

“One evening I was scared, when there were some police officers visiting us in the night, claiming they needed some blankets for some other women; there was fear of some mischief,” said Mtetwa, who was beaten during her 2003 arrest. “But everybody knew I was inside, and I was not really afraid; it was just an inconvenience to spend eight days cooling your heels.”

Her arrest was widely seen as a crackdown ahead of the presidential elections scheduled for this summer. The last time Mugabe stood for reelection, in ’08, opposition supporters were raped, kidnapped and killed. Although the president lost, the result was effectively voided. This time, Mtetwa says,  she’s hopeful that international pressure and observers will prevent violence, though it’s hard to understand where that optimism comes from.

Here in Cambridge, where I saw a screening of the film last week, it’s been hard to think beyond the pain inflicted here by the marathon bombers. But even through that prism, I invite those who saw the FBI’s initial decision not to read 19-year-old suspect Dzhokhar Tsarnaev his Miranda rights as evidence that we live in a “police state” to see this movie for some perspective on what a real police state looks like.

There’s also some unexpected solace for me in the movie as I keep trying to figure out how the younger Tsnarnaev, who was so widely loved during his years at the high school my son now attends, could have wound up accused of using a weapon of mass destruction, then partying and hitting the gym like nothing had happened.

One of Dzhokhar’s mentors, a high school wrestling coach, has told reporters that he’s broken-hearted to think that the boy he was so fond of had turned all the care and guidance he tried to give him against us all.

But Mtetwa is just as mysterious and powerful a force for good; the oldest daughter of a man with six wives and more than 50 children, she managed to seize on the encouragement of one teacher who told her she had some potential, and never let go.

And instead of hardening, as Dzhokhar apparently did, Mtetwa turned all the hardship in her own life into a gift to those who’ve needed defending. “In my entire life, I had to deal with imposing, difficult males, starting with my father,” she says in the movie. “Despite him being physical when we didn’t agree, I never actually stopped challenging him.”

When I asked whether she feared not being allowed back in the country after her trip to Washington, she didn’t really say: “Legally, I haven’t done anything wrong, but who cares about the law?” The answer, of course, is that she does: “This has to be done,” she says of her work, in the last line of the movie. “Somebody’s got to do it, and why shouldn’t it be you?”

Melinda Henneberger is a Post political reporter and anchors She the People who is spending this semester as a fellow at the Harvard Kennedy School’s Shorenstein Center. Follow her on Twitter at MelindaDC.