Can a sudden religious conversion be convincingly dramatized? Can a man who has done cruel, hateful, casually sadistic acts as a young 18th century British slave trader for almost three quarters of a musical become a sympathetic hero by abruptly discovering his faith? Is it possible to excuse some of history's worst crimes against humanity by explaining that, well, the perpetrator was merely a brat rebelling against his father for neglecting the boy's pious, dying mother?

And while we're asking, is it possible today to tell another slave history through the eyes of a white man and the noble woman who believes in him?

So much effort, sincerity and talent -- not to mention cash -- have been funneled into "Amazing Grace" that it would be nice to be able to answer yes to any of that.

Alas, the lavish historical epic may be inspired by the real life of John Newton, the slave dealer who saw the light, became a church leader and wrote what would become one of the world's most popular songs of redemption and forgiveness. If so, however, the truth is far too big for this Broadway package.

This is not to suggest that the show -- created, composed and co-authored by first-time writer (and former Philadelphia police officer) Christopher Smith -- is short on the spectacle, anthems and power ballads that suggest admiration for "Les Miserables." The production has been confidently directed by Gabriel Barre, with a first-class creative team and a large, excellent cast -- most impressively, Chuck Cooper and Laiona Michelle in supporting roles as the families' favorite slaves.

We travel through big storms on huge ships, suggested by high masts and crow's nests, from England to Sierra Leone to Barbados. There is even a gorgeous underwater scene in which Newton is saved by his loyal lifelong servant (Cooper).

Josh Young croons passionately as Newton, but, finally, is incapable of making this monster sympathetic. Erin Mackey has a spiky temperament and soprano to match as his childhood love. In Africa, there is a campy-evil black "princess," a slave trading woman just as bad as the white guys, and a lovely girl (Rachael Ferrera) who appears in absurdly coincidental plot points.

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But what does all this have to do with "Amazing Grace," the song? Not much. It is heard at the beginning of the show and in an audience sing-along at the end. Mostly, this is Newton's backstory -- an adventure saga with morals about slavery, women's rights, abuses by the Empire and dysfunctional family melodrama. As for the genuine religious conversion, we must take it on faith.

WHERE Nederlander Theatre, 208 W. 41st St.

INFO $65-$139; 877-250-2929;