The Sentence isn’t a great film. But it’s a strong film, with an even stronger message: that tough laws aren’t always smart laws.

The Sentence, which airs Monday at 8 p.m. ET on HBO, tells the story of Cindy Shank (top). She was convicted of conspiracy to deal drugs because her now long-dead boyfriend had been a drug dealer, and while there was no indication she was part of his operation, the “mandatory minimum” laws required the judge to give her 15 years in prison.

At the time of her trial, the boyfriend had been dead for six years. Shank had married a guy who wasn’t a drug dealer and now had three young daughters. What the 15 years really meant, she says in the film, is that she was sentenced to miss all of her daughters’ growing-up years.

Shot by Shank’s brother Rudy Valdez (top) and winner of the audience award at Sundance, The Sentence doesn’t argue that the conviction was a miscarriage of justice.

Rather, it makes a single argument: that mandatory minimums, which were instituted in the 1980s amidst the political posturing that surrounded the “war on drugs,” do nothing to stop dealing. Instead, they prevent sentencing judges from administering balanced justice because those judges are prohibited from weighing the individual circumstances of a case like Shank’s.

Shank, whose recorded conversations are a critical part of the film, admits she knew her ex was dealing drugs. She stayed with him, she says, in hopes she could get him out of that world and bring back the sweet guy she first fell for.

It’s an oft-told tale, one imagines. In the legal biz, it even has a nickname: “the girlfriend problem.” But the mandatory minimum clauses of drug legislation don’t differentiate between the perpetrator and everyone else who lived in the house, whether or not there’s evidence of collaboration.

In any case, Valdez says the film started when he took some home video of Cindy’s kids so he could send it to her. Quickly, he says, he decided this footage was telling a bigger story.

The Sentence is still rooted heavily in his videos of the kids, whom we follow as they grow up through the years of Cindy’s confinement.

Purely as cinema, a lot of it really does look like home video shot on a smartphone. Not to sound callous, there’s also a little more footage of Cindy’s kids than might feel enthralling to anyone who isn’t Cindy.

The kids are adorable and seeing them speak underscores the deep mutual sadness of being separated from their mother. Their father does all the right things, and still, there’s that big hole.

After a while, however, the long stretches of family film and conversation feel as if they’re reemphasizing the same point. They also give The Sentence the feel of a passion project, which it justifiably is, rather than a polished documentary.

There are good and bad sides to that, and we also should remember this is Valdez’s first documentary, one he put together almost as an afterthought.

In any case, he delivers his message. When a campaign begins to get early release for Cindy, we’re rooting for it to succeed, particularly after we’ve seen the internal workings of the prison system. While Cindy’s kids live in Michigan, she gets transferred at one point to a prison in Florida, which makes visitation almost impossible and seems unnecessarily harsh.

Mandatory minimums were born out a widespread national consensus that we need have no sympathy for drug dealers. The Sentence reminds us that even within that problematic universe, we need to have flexibility if we’re also going to have justice.