It’s a strange, seesawing time for labor rights and union power. On the one hand, workers’ concerns are hitting new highs across many sectors, while the Republican presidential candidate barely made waves on the campaign trail for insulting autoworkers by saying a child could do their job.

Union organizing has been on the move, however, and no cause seemed more thrilling than the grassroots movement Chris Smalls co-founded in 2021 to organize Amazon employees at the Staten Island warehouse from which he’d been fired. And if getting canned sounds like some reputational disqualifier, what you’ll learn from “Union,” the gripping, on-the-ground documentary from Brett Story and Stephen Maing, is that the perennial axing of employees for specious reasons is key to the trillion-dollar behemoth’s bottom line. Disposability is how Amazon maintains draconian control over its workers’ fragile sense of job security, leeching their desire to push back by unionizing.

But with Amazon also playing heavily in the entertainment sector, does that fear-inducing corporate power explain why one of the most celebrated documentaries to come out of this year’s Sundance Film Festival couldn’t secure a mainstream release and is now being self-distributed? Most assuredly it does, the filmmakers of “Union” contend. Watching footage of Smalls and his fellow volunteer organizers strategize against the virulently anti-union Amazon — manning sign-up tents in freezing weather, crashing coercive union-busting meetings, even giving away free weed — carries an extra resonance. “Union” feels like a truly independent movie that was in the right place at the right time to witness one of the biggest labor victories in generations.

But that also means laying bare all the tensions that build when eager activists from different backgrounds, with ideas and egos but no playbook and no national union backing (not that they don’t try), have to learn how to stay focused and motivated as the struggles mount. The filmmakers are firmly on the side of labor, as Barbara Kopple was in the early 1970s chronicling a miners strike in her iconic documentary “Harlan County, USA.” And outside of a police altercation that rattles one sensitive organizer, thankfully nothing like the gun violence seen in Kopple’s film happens here.

But you’ll feel heavy-hearted when faith falters, insensitivities are alleged and one vigorous early supporter senses defeat and ultimately votes against forming the union.

Recurring images of a massive barge stacked high with shipping containers send a curiously two-sided message here: Amazon will continue no matter what, but so will efforts to unionize. The good news that ultimately made headlines in 2022 is justly celebratory, but the subsequent difficulties we see heighten the notion that a union needs cohesion, direction and sustenance beyond a star organizer having a well-earned moment.

Smalls is a decidedly charismatic rabble-rouser in the momentous year “Union” chronicles. But in a quieter moment, reflecting on when he was fired after trying to hold Amazon to COVID protocols, he articulates something that gets forgotten in the nuance-free David versus Goliath rhetoric: The goal is for everyone, from top to bottom, to thrive. “They didn’t realize how invested I was in the company,” he says, almost wistfully. Sober and heartfelt, “Union” lets us see what Amazon and the world would soon discover about the power workers have when they invest in their dignity first.