For American moviegoers, Amazing Grace fills an amazing gap in popular knowledge. We may leave the theater wondering why its hero’s name, William Wilberforce, has not become a byword for stubborn courage and liberating zeal, the way Frederick Douglass’s is. Is it simply because this eighteenth-century British politician and antislavery crusader is part of England’s history rather than ours? Or is it that the elimination of the slave trade in England didn’t entail vast battlefields of corpses, and therefore isn’t as immediately movie-friendly as our Civil War?

Although I have heard some historians complain that the film bends facts to exalt Wilberforce’s name at the expense of equally important abolitionists, an undeniably stirring story unfolds. Our hero at first tries to feed his strongly religious nature with solitary meditation within the safety of his comfortable country estate but is drafted into politics by his friend William Pitt, whose Whigs are battling the Tories in Parliament. (One drawback of this movie’s narrow focus is that we never understand what Pitt’s platform encompassed. The Whigs come across, improbably, as reformers whose only concern was the antislavery campaign, with Wilberforce as their champion.) The first abolition effort fails when the Tories brand the liberals as subversives during a time of conflict with France, such words as “liberty” and “equality” being tainted by the excesses of the French Revolution. (Shades of the last American presidential election, with Democrats labeled as soft on terrorism if they voiced a concern about civil liberties.) Yet, supported by the love of a good woman and the fervor of his ex-slaver cousin John (author of the famous hymn “Amazing Grace”), and covertly encouraged by the more cautious Pitt, Wilberforce perseveres and prevails.

This story, written by Steven Knight and directed by Michael Apted, depicts liberalism at its most heroic, so I think it’s safe to predict that as a DVD Amazing Grace will be shown in high-school and university courses, at religious convocations and retreats, and indeed at any gathering that studies the history of slavery, imperialism, or the possibilities for racial harmony.

The worst thing that can be said about Amazing Grace is that it often seems determinedly geared toward such gatherings. The problems and issues raised by the film are presented too simply; the admirable characters are too perfect, the villains too hissable or laughable (I’m talking here about the writing, not the acting); the thrust of the narrative is engineered all too obviously to make us feel good about identifying with the good. I’m not suggesting that the movie should have been a deflation, an exposé, or a psychodrama. But, for me, heroism is intensified by an acknowledgement of the hero’s darkness. Lancelot is more vivid than Galahad because Lancelot sins and yearns while Galahad only prays and wins. Was Wilberforce never abrasive in his single-mindedness or overweening when he scented victory? The moviemakers don’t even allow him his quirks, such as his disapproval of Pitt’s ordering soldiers to drill on Sunday. The danger of keeping psychological shadows out of the portrait of a hero is that you wind up with a mere role model instead of a true hero.

There is another danger to which the film occasionally succumbs: that the evils the hero combats will simply not seem evil enough if presented in too digestible a manner. When Olaudah Equiano, an ex-slave abolitionist, brings Wilberforce to the bowels of an empty slave ship in order to certify the horrors of the business, we know intellectually what the scene is supposed to accomplish. We are to feel with Wilberforce the misery dripping from the walls and evoked by the vestigial cries of the slaves. But the staging doesn’t accomplish this. The words given to Equiano are a mere laundry list of horrors; the lighting of the nautical dungeon is too artily modulated, the shadows satiny, the illumination mellow. In its lack of pungency and real horror, Amazing Grace earns its PG rating too well.

Yet the movie finally becomes compelling in its second half because it gets into a subject of real complexity, one rarely addressed even by films that purport to deal with politics: what George Will has called “soulcraft through statecraft,” the enforcement of justice through the mechanics of legislation. All too many movies, especially American ones, present idealistic politicians achieving their goals through sheer fervor, as if there weren’t procedural hoops to leap through. Scriptwriter Knight shows that even the good guys must sometimes resort to questionable tricks to make virtue prevail. That this doesn’t appear to diminish Wilberforce and his colleagues is testament to the way Knight helps us understand the expediencies required by the rough-and-tumble of politics.

Of course, it helps the sheer entertainment side of Amazing Grace that the British Parliament was and remains just as much a place for theater as for dealmaking. (Just turn on C-Span Sunday nights and watch Tony Blair and his opponents rend one another amid an unceasing cockpit uproar that makes our Congress look like preschool story hour.) This requires that actors playing British politicians, particularly eighteenth-century politicians, be able to revel in the words they wield as weapons. Ioan Gruffudd as Wilberforce, Benedict Cumberbatch as Pitt, Albert Finney (the ex-slaver cousin), Ciarán Hinds (the Tory leader), Toby Jones (the Duke of Clarence), and Michael Gambon (Lord Fox) all roar and rumble, cajole and denounce to excellent effect.