The most implausible aspects of Kasi Lemmons’s laudable Harriet Tubman are all historically accurate. Using the North Star, Tubman walked 100 miles to escape from slavery in Maryland in 1849 only to return, via the Underground Railroad, to rescue other family members.

Armed with a pistol, she made that same dangerous trek from north to south more than a dozen times to free others, earning the nickname Moses. During the American Civil War, she and her Union army unit were involved in a daring military operation that freed more than 750 slaves.

And yes, she was guided by narcoleptic visions which, by her account, were messages from God. (The film’s insistence on Harriet’s religiosity may be factual but it does take some of the glint from her considerable steeliness.)

Kasi Lemmons, the director of Eve’s Bayou and Black Nativity, is careful and sincere in her appraisal of her historical heroine. The Oscar-winning cinematographer John Toll (Legends of the Fall, Braveheart) delivers some of the most epic tableaux of his career. English actor Cynthia Erivo puts in a performance that is as spirited as it is moving.

The script, co-written by Lemmons and Gregory Allen Howard, contains a few contractions that don’t sit well, notably an Oscar-baiting speech delivered to a room full of abolitionists (including Frederick Douglass). Likewise, Tubman’s relationship with her fictionalised “owner” (Joe Alwyn) builds to a ludicrous stand-off, replete with white horse.

Despite these occasional missteps, Lemmons mines plenty of thrills and fervour from Tubman’s extraordinary adventures. If anything, and in common with many biopics, there’s simply too much incident to cram into two hours. Any one of the film’s subplots – Harriet’s first 100-mile walk to freedom or her military activity during the civil war – might have made for a neater, niftier movie.

This is a fine and essential primer, nonetheless.

Harriet is released on December 22nd