A devout man of firm conscience who mixed evangelical zeal with political savvy, William Wilberforce, shook up Britain's parliamentary system in the late 18th century - and galvanized a good portion of the British public - with his long and determined crusade.

In Amazing Grace, that fight - against slavery, and against the shipping and trade industries in Britain that relied on slavery for their commercial well-being - is depicted in simple (but not simplistic) strokes, with the accordant powdered wigs, horse-drawn carriages, stately mansions and fancy dress of a self-respecting period drama.

Sturdily directed by Michael Apted, from a screenplay by Steven Knight, Amazing Grace stars Ioan Gruffudd as Wilberforce, presented here as a reluctant pol who had to be swayed from the priesthood so he could fight the good fight in the corridors of power. Allied with William Pitt (Benedict Cumberbatch - there's a name!), destined to become the youngest Prime Minister in British history, Wilberforce lobbied, strategized and eventually out-flanked a powerful band in the Houses of Lords and Commons who supported slavery for the economic good of the land.

Wilberforce's chief opponents, and Amazing Grace's chief villains are the contemptuous Lord Tarlton (Ciaran Hinds) and the Duke of Clarence (Toby Jones, who played Truman Capote in the lamentably little-seen Infamous). Albert Finney appears briefly as John Newton, a Wilberforce mentor beaten down by politics and corruption, and now living as a monk. Rufus Sewell, eyes ashine and hair a-long, plays a nutty grassroots abolitionist, while Youssou N'Dour, the African music star, is a freed slave whose published accounts of his ordeal helped to galvanize the anti-slavery movement.

Framed as a flashback, and a romance, Amazing Grace unfolds as an older, wearier and not altogether well Wilberforce describes his parliamentary maneuverings to a lovely lady (Romola Garai) whom he's been set up with by friends. She's passionate about politics and just causes, too, and as they stroll the gardens and share a divan by the fire, the film flips back to the story at hand.

Apted opts not to show the horrendous cruelty inflicted on thousands upon thousands of captive Africans, shackled and chained, making their way to the Americas in ships. Instead, he has Wilberforce and his fellow abolitionists describe the inhumane conditions - in the precise, passionate language of legislators who believe that human decency is more important than money and power.

Amazing Grace *** (out of four stars)

Produced by Edward Pressman, Terrence Malick, Patricia Heaton, David Hunt and Ken Wales, directed by Michael Apted, written by Steven Knight, photography by Remi Adefarasin, music by David Arnold, distributed by Samuel Goldwyn Films.

Running time: 1 hour, 56 mins.

William Wilberforce........Ioan Gruffudd

William Pitt. . . Benedict Cumberbatch

Barbara Spooner. . . Romola Garai

John Newton..................Albert Finney

Olaudah Equiano. . . Youssou N'Dour

Parent's guide: PG (adult themes)

Playing at: area theaters

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Contact movie critic Steven Rea at 215-854-5629 or srea@phillynews.com. Read his recent work at http://go.philly.com/stevenrea.