The quality of the documentaries at Sundance London this year has been delightfully high, comprised of fresh directors breaking out boldly (Dinosaur 13) and old masters continuing to prove their mettle (Finding Fela!). While The Case Against 8 doesn’t quite fall into either camp, it still holds the power to move you to tears.

Back in 2008, a time which now feels oddly distant, a major step backward for human progress showed itself in the form of Proposition 8, a ban on same-sex marriage in the state of California, voted for by the state’s people. How so many people can be so unforgivably wrong is another matter, for this movie – shot over five years – is concerned with telling the tale of how that wrong was ultimately righted. We focus on two gay couples; the first are two women who were previously married, have four children between them, and the second are two men who wish for marriage – and potentially a family themselves – one day. These people not only make a perfect case for the campaign against Prop 8, but serve as the documentary’s anchor; they are always the focus, reminding us what the entire fraught affair is about, even when the thrust is directed inside courtrooms or at TV propaganda.

Basing the movie on this colossal subject is a brave, and ambitious, choice by co-director Ryan White, this being only his third feature, and even braver for Ben Cotner – this being his first. But the two weave together what must’ve been countless hours of footage from the families’ homes, the offices in which the fight against Prop 8 is planned, and much press video, into an involving and deeply moving indictment of injustice of not simply the political kind, but the sort that transgresses basic human rights. Despite the film’s wise choice of keeping the struggles gay people go through every day at its core, the doc’s curious declination to delve deeper into the complexities of the legal proceedings – which are at the centre of not just the families on screen, but every family in America – means that the doc lacks a particular kind of authenticity that would act to enlighten us further on the subject. There’s also not much thought given to the other side; no matter how wrong we think those who supported Prop 8 are, surely their opposing views would’ve given more colour to the argument, instead of building what is essentially an extended victory call? Nevertheless, wherever Cotner and White aim they always hit – which is still a satisfying amount given the timespan of the campaign, even though there’s always the sense that there’s something missing.

We all know how The Case Against 8 ends, but you’ll still be moved to tears by the movie’s ability to depict a world in flux through the changes in a handful of people’s lives. It’s also extremely enlightening to those who don’t live in the States – which happens to be a lot of people – and who may not have followed the case in detail. Despite its flaws, The Case Against 8 articulately, and triumphantly, shows the world being changed by the minority; if that doesn’t get you out of bed in the morning, what will?