There's much to be astonished by in the story of how the Supreme Court was goaded into slapping down Proposition 8, California's gay-marriage ban. One of the most surprising: In courtroom after courtroom, be they state, district or superior, Charles Cooper and the proponents of the ban never cooked up one single compelling legal argument, not even the kind that just sounds good enough for sympathetic judges to sign on with.

Ben Cotner and Ryan White's stirring documentary, The Case Against 8, showcases the lawyers and plaintiffs who came together (with Rob Reiner's money) to challenge the ban, which passed as a ballot measure in November 2008. The filmmakers target the heart rather than the complex legal maneuverings. Expect to spend more time relishing the courage of the plaintiffs, two charismatic California couples, than pondering the Constitution's equal-protection clause or the issues of standing that the case ultimately turns on. This is a popular history, which trembles with tears and hope, and I dare you to get through it without bawling yourself.

That's no betrayal of the wrangling of the plaintiffs' ace legal team, headed up by the strangest of bedfellows, David Boies and Ted Olson, the attorney avatars of (respectively) blue and red America, who squared off before the Supreme Court in Bush v. Gore. (Olson won that one.) They are fascinating characters, but we rarely see them before a judge. In most of the proceedings, cameras were banned from the courtroom, so the directors resort to footage of the plaintiffs reading from their own testimony after the fact.

That move turns out to be revelatory. As they declaim their own words, often professions of deep and abiding love for their partner, we can imagine the drama of the trial we're missing. We're also treated to something grander: regular folks revisiting the plain language with which they changed the world, each clearly moved and humbled, each understanding that a historic wrong was righted by their own love.

That's not to say that crackerjack lawyering wasn't essential, despite the fact that the proponents of Proposition 8 utterly failed to find precedent for the systematic denial of civil rights to a minority population. Defending the indefensible, the anti–marriage equality crew hauled out the usual think-of-the-children nonsense, and they nattered on that the long history of state control of marriage has actually been about ensuring hearty levels of procreation, which makes as much sense as a case against Rush Limbaugh's fourth marriage as it does against any gay person's first.

Defending Proposition 8 at the state level, David Blankenhorn, their expert history-of-marriage witness and founder of the Institute for American Values, got so tangled up under Boies' adept cross-examination that he wound up agreeing, right there on the stand, that marriage equality would actually be more American than its opposite. Blankenhorn has since fully converted, and his mea culpa is another welcome surprise here. Many of us wear “a tissue” of certainty that “keeps you from seeing other people,” he argues in a fresh interview. The idea is that he's torn away his tissue, but he looks a little prouder of this metaphor than he should for a guy who only stopped trying to block marriage equality after he got his clock cleaned.

Highly entertaining and beautifully human, The Case Against 8 is certain to tear that tissue away for many viewers who stumble upon it once it premieres on HBO on June 23. The film isn't the deep dive into behind-the-scenes strategizing you might be hoping for: The camera crew gets doors closed on it at key moments, and on occasion the directors overplay the drama, building to courtroom showdowns they're then unable to show us. We get an idea, though, in the many scenes of rehearsals for the trial, in snatches of videotaped depositions and in the tender, resonant interviews with the four plaintiffs, whose lives would have been upended by the case even without the death threats.

The final half-hour is a flurry of appeals as the case moves between courts. The Supremes are not quite the finale — the upshot of that 5-to-4 decision is left vague until a final title card — but the court itself is a thrilling climax. The plaintiffs and their team trot up the grand stairs, pressing through protesters and Westboro Baptist Church types. But the cries of the haters are drowned out by the roar of the rest of us, the Americans who are so heartened to see that, at long last, the tissues are coming off so many faces.

Have some tissues of your own ready for the end. If you're a person, the weddings will melt you.

THE CASE AGAINST 8 | Written and directed by Ben Cotner and Ryan White | HBO Documentary Films | Royal

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