Harriet focuses on a crucial few years in the life of Harriet Tubman, a self-sacrificing, heroic, Civil War-era woman, nicknamed “Moses,” who braved obstacles and dangers not only to free herself but to continually return to the South to deliver slaves from bondage.

The film is not a full-fledged Tubman biopic. Rather, it explores the motivations that drove her to continually risk her life, as well as details the strengths that propelled her — her faith and spirituality.

As a child, Harriet suffered a head injury. After she recovered, she began having spells during which she saw future events. She interpreted these visions as messages from God. And it was her belief and certainty that she was doing the Lord’s work that drove her mission. Harriet, directed by Kasi Lemmons from a script by Gregory Allen Howard and Lemmons, uses Tubman’s conviction as its foundation.

Tubman and her family were the property of a Maryland farmer before she fled the 100 miles to freedom in Pennsylvania. There, she was given an education, and, a year later, decided to return to Maryland to free her husband and other family members.

Harriet is undercut by too many moments that approach melodramatic tropes and stereotypical characters rather than fleshed-out people. The movie, however, is a vehicle for Cynthia Erivo, who brings a steely determination, resiliency and stubbornness to her portrayal. Erivo’s Harriet is confident that she is following God’s will, and she won’t allow anyone or anything to deter her from her goal of emancipating as many slaves as possible.

Harriet rests on Erivo’s shoulders, which offers short shrift for the rest of the cast members, among them Leslie Odom Jr. and Janelle Monáe, who help run the Underground Railroad, and Vondie Curtis-Hall as a black preacher with a hidden agenda.

Joe Alwyn, who plays Gideon, the slave owner who wants Harriet back, is a one-dimensional figure who plays his part as if he just stepped out of a production of Mandingo. The sexual tension when he is onscreen is a trite throwback that is an unnecessary plot component.

Smartly, Erivo is able to use her vocal talents — she starred as Celie in the 2015 Broadway revival of The Color Purple — singing spirituals as signals to slaves that she was at hand and ready to lead them North.

The movie — no pun intended — is too black and white. Most of the characters lack complexity; they are either good or bad, and many situations are formulaic sequences derived from other films that touched on the issue of slavery. Harriet is definitely worth seeing, especially because of Erivo’s bravura and impassioned performance. It’s a shame that the script fails to reach her heights.