PARK CITY - You could be forgiven for wondering what we stand to gain from a documentary about the Proposition 8, the ban on same-sex marriage issued in California in 2008, and its subsequent reversal. The final Supreme Court ruling on the case is not even a year old, and the five long years of back-and-forth legal wrangling toward this resolution should be fresh in the minds of even those only casually concerned by the issues at stake. Has enough time passed for our perspective on the events to have shifted? Are proponents of the ban ready to engage in even-handed conversation? Can any film on Proposition 8, whatever its stance, do much more at this stage than preach to the converted?

The answer to those questions is probably no. And yet Ben Cotner and Ryan White's "The Case Against 8," a straightforward but exhaustively detailed chronicle of the case from inside the crusading legal team, seems a significant, even necessary film -- one, perhaps, of even greater long-term than immediate value. The Sundance audience that saw the film today applauded with the satisfied vigor of those still high from a liberal victory. Future generations, however, will hopefully regard it with grim fascination, aghast that the case ever took place to begin with, and in the 21st century to boot. "The Case Against 8" doesn't indulge in lumpy rhetoric or editorialization; as a methodical document of the battle against Proposition 8, in its many, many stages, it's stirring enough.

Which is not to say it's lacking in humor or human spark. In its observation of the legal team's camaraderie, not to mention the enduring love between the two steadfast couples -- David Zorrilla and Paul Katami, and Kris Perry and Sandy Sterm -- selected to symbolically spearhead the campaign, "The Case Against 8" may well draw up the blueprint for an uplifting narrative feature on the very same subject. Its chief redemptive twist: the enlistment of crusty conservative old-timer Ted Olson, former attorney-general to George W. Bush, to lead the charge. (And with his his old Washington opponent David Boies as his co-counsel to boot.) Sometimes real life simply serves up good stories, and this is one.

First-time filmmaker Cotner, a former acquisitions VP at Open Road Films, and the more seasoned documentarian White have taken a clean, linear approach here: granted extensive inside access by eager activist Chad Griffin and the American Foundation for Equal Rights, the development of the case is mapped out with the brisk momentum of a let's-put-on-a-show musical, from the casting of spokespeople to the emotionally testing rehearsals to the heart-in-mouth rush of the final performance. This might have been a different, drier film if the team's initially successful request to have the courtroom proceedings televised hadn't been overturned.

As it is, Cotner and White hit on an artful alternative: to have principal participants re-enact their testimonies and legal arguments on camera, a moving, self-effacing technique that also draws attention to the structural elegance and resonance of so much that was said in that courtroom: Boies' questioning, in particular, deserves repeating and preservation for posterity: a series of crisp logical pretzels designed to prove the impossibility of inherent harm in any legally sanctified LGBT coupling. His effective conversion of former hardline "Yes on 8" proponent David Blankenhorn is particularly stunning.

"The Case Against 8" works, then, as a clear-eyed insight into the nuts and bolts of legal and political process, but impresses with its deft incorporation of personal anguish and catharsis -- much of it free of any intervention or construction on the part of the filmmakers. A bare, spontaneous confession from Kris Perry during a witness training session, in which she tearfully recounts a lifetime of stoically endured prejudice and concludes that "coping is a really low bar to set," is typical of the devastating rewards the film reaps simply by pointing its camera at the right people in the right circumstances.

As we all know by now, "The Case Against 8" has a happy ending, something the filmmakers could hardly have counted upon when they embarked on this project. The film's emotional rewards, however, would be rich even if there hadn't been one: those who still regard gay marriage as an aberration might not have their minds changed by anything that is said here, but they'd be hard pressed to deny the resilience of romantic partnership on display. "Marriage has lasted because it has changed," says Boies at one point. We can only hope the most recent change to the instituion is a permanent one.

Guy Lodge is a South African-born critic and sometime screenwriter. In addition to his work at In Contention, he is a freelance contributor to Variety, Time Out, Empire and The Guardian. He lives well beyond his means in London.