The “mystery” documentary is a tough genre subset. First off: Provided that the film doesn’t involve unsolved violent crime, is it truly that mysterious? Is a myopic filmmaker’s perspective rendering that molehill a mountain? If it is actually mysterious, the battle royale begins between mood, momentum, meaning and, most of all, the ethics of honest storytelling.

Sometimes, the mystery really works (Tickled). Sometimes it sort of works (Searching for Sugar Man). Most of the time, it’s overblown (Three Identical Strangers) or outright ludicrous (Catfish). So it’s natural to feel trepidation as Sam Now purports that a parcel of home-movie productions became “the key to unlocking the biggest mystery of (the Harkness) family.”

From the late 1980s into the early 2000s, Reed Harkness called the shots on bloody, wacky and largely improvised Raimi-esque films around Seattle starring Sam, his half-brother from another mother. As Reed says, he and Sam were always running out of film, gas or sunlight. A gangly kid whose clumsiness earned him the nickname “Candy Bones,” Sam eventually created a superhero alter ego for himself known as the Blue Panther. It is for this character that Reed, older by several years, suggests the ultimate mission in 2003: What if the Blue Panther tried to find his mother?

The Blue Panther’s mother is, of course, Sam’s own mother — Jois, who hung around for a couple of “normal” years after divorcing Reed and Sam’s father before she dropped off the map without warning — or any further contact — in 2000. Reed sparks his plan because no one in his family really even seems to talk about Jois’s disappearance. So begins a film that turns out to be 20 years in the making, as Reed and Sam interview family and friends about what they know (or believe) to have happened, discover a trail they hope will lead them to Jois and, ultimately, confront generations of emotional isolation and manipulation that besets their family.

While Sam Now — which premieres Monday in PBS’s Independent Lens series — takes its title from one of Reed and Sam’s throwback films, the documentary comes to address many splintered branches of the Harkness family: the preference of patriarch Randy to shut down and scuttle serious discussion of Jois’s disappearance; the minefields created for Sam’s biological brother, Jared, after Jois leaves; the way Jois’s child from a previous marriage plays off her absence as no big deal, perhaps as a front for his own frustrations; and a grandmother who recognizes more personality disorders than she lets on but also would like for the family to avoid all of this fracture.

As for the mystery of Jois’s whereabouts, there is a definitive resolution before Sam Now hits the halfway point — and, at least relative to its buildup, a deeply anticlimactic one. At least what happens leads to more compelling engagement, whether hurtling forward through time into Sam’s life as an emotionally stunted adult or pulling at a frayed, international thread from Jois’s childhood. Questions remain, though, of whether the Harknesses are really exorcizing feelings and evolving their emotional responses to Jois’s decision … or if this is all just somewhat of a series of exploitative gotcha moments in hopes of capturing NASCAR pileup-level family drama. (Story-credit consultants perhaps suggest a bit too much mucking around with the details.)

While the notion of patterns playing out across the Harkness family history is persuasive, it doesn’t hit with the oomph Reed Harkness wishes it would. At least there is no easy resolution to that offered in the closing, which would have felt orchestrated for obvious closure. Although certainly oversold as a mystery, Sam Now derives decent power from its consideration of the intersection of family and filmmaking — a fine line between peaceful memories worth preserving and those we pervert into something far more comforting and fanciful than they were in reality.