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"God showed me the way," is how Harriet Tubman explains the astonishing feat of leading dozens of slaves to freedom. If only God could have done a solid for the biopic's screenwriters and composer.

The good things in "Harriet" are constantly undone by Terence Blanchard's old-fashioned, overbearing music, which underscores (but really overscores) the action as insistently as in the silent-movie era. And the Gregory Allen Howard-Kasi Lemmons screenplay relies on explanatory dialogue that makes it seem less like the characters are talking to each other and more like they are looking out in the audience to make sure we're up to speed on what's happening.

Cynthia Erivo's forthright performance as Harriet is the movie's biggest asset. Sure, it stretches credulity that Tubman possesses not just an iron will but also a Tony Award-caliber singing voice. (Erivo did win a Tony for "The Color Purple.") But, in every other way, Erivo grounds the character in humanity. Her Tubman is a modest woman who credits God for all of her achievements. Quite rightly, the movie insists that Tubman is one of our greatest heroes, but even when she's making a Big Speech, Erivo plays Harriet as calm, determined and confident that faith is guiding her.

One of the movie's most interesting ideas is that slavery created Tubman's superpower.

"You've been free so long you forgot what it's like," she tells a fellow conductor on the Underground Railroad.

Harriet, who escapes shortly after the movie opens, has not forgotten, and the strength, endurance and piety she used to help her get through every difficult day serve her well when she goes to work to lead others to freedom. I'd have loved more detail about how the Underground Railroad functioned, but "Harriet" offers fascinating glimpses of, for instance, spirituals being used as a code and kind strangers finding ways to make themselves known to escaped slaves.

Unfortunately, there's not enough of that level of complexity in the screenplay, and by making Tubman's achievements seem relatively trouble-free, it undervalues the danger and impact of her work. Too often, the script takes the easy way out with huge coincidences and cliches.

"We're going to need a bigger cart," a line spoken by an accomplice who learns the number of slaves being carried has just doubled, is either a really inappropriate "Jaws" joke or a really dumb accident. Either way, it takes you out of the movie. I wish, too, that "Harriet" were stronger at providing context when the Fugitive Slave Act of 1850 makes Tubman's work more difficult by insisting that, essentially, there's no such thing as a freed slave, even in the North.

"Harriet" is a necessary movie — it's shocking it has taken this long for a project on this scale — and, as a director, Lemmons atones for some of her screenplay's flaws. Lemmons covers a broad swath of Tubman's middle years, always organized so that we're clear on where we are and when the action is happening. Along the way, Lemmons makes room for complexity with a few unexpected scenes, including one in which a friend admonishes Tubman not to judge her for deciding to stay under the yoke of slavery and another that lays bare the foolishness of slave owners thinking they could mistreat their "property" and expect loyalty in return.

Most important, Lemmons chose Erivo, a movie newbie who is British and not a box office name but who is exactly the Harriet that "Harriet" needs.