The story is very freely based on the life of Harriet Tubman, a slave who escaped from bondage on the Eastern Shore of Maryland, and who became a "conductor" on the Underground Railroad, returning back nineteen times and rescuing over three hundred of her people. At one time a reward of $40,000 was offered for her recapture alive or dead.
The opera opens with the slaves and Harriet passionately crying out for freedom. They all fear what will happen when "Old man buzzard comes to visit the big house" because their ‘ole Marse’ is indeed old and sick and his son Preston has just arrived home: it soon becomes clear that Preston is as irresponsible as he always was.
Harriet meets her friend Josiah and they speak of their love and their plans to get married.
Entrusted with Preston's baby daughter, Harriet sings her to sleep, a lullaby based on "Swing Low, Sweet Chariot". Preston enters and tries to seduce her. To his fury she successfully defends herself.
Rit tries to comfort her daughter, who is distraught from the confrontation with Preston. But she can offer no solution to injustice, only resignation. "It ain't right! It ain't justice! But that's the way it is."
Preston confesses to his father that he has incurred a huge gambling debt. The Master explains he cannot possibly help. He is appalled by his son's suggestion that he should sell some of his slaves to pay off the debt. But under pressure from the local slave trader and leader of the patrol the Master is forced after all to sell some slaves to pay off the debt. When Josiah hears he is one of those to be sold South he decides to run. He has heard of the Underground Railroad that allows slaves to make the dangerous journey North, about "the train that leads you out of bondage, out of Egypt, to the promised land," and about "the white folks willing to help." Harriet bids him a hurried farewell and promises to follow him North very soon where they will be able to live their lives in freedom.
Harriet soon follows for she realizes that Preston will soon be her Master. She resolves that she will never be his slave; she will escape or die in the attempt. "There's two things I got a right to, these are liberty or death. One or the other I mean to have. No one will take me back alive. I shall fight for my freedom."
Once in the North Harriet only seems to hear the voices of her people calling out louder and louder for help. She realizes that she must go back South and, like Moses, deliver her people from bondage. She is frightened by the overwhelming responsibility of what she is asked to do.
Harriet soon becomes an experienced conductor on the Underground Railroad. Though her identity is a closely guarded secret, this “Moses of her people” eludes the slave Patrol in a series of escapes that have become increasingly dangerous.
Harriet learns that the newly passed Fugitive Slave Law even allows houses in the North to be searched for runaway slaves and that Harriet and all the others who have escaped will now have to flee to Canada. She also learns of the death of her “ole Marse”: she refuses to think of her own safety and goes back South one last time to rescue her parents.
Meanwhile at the farm everybody mourns the passing of the ‘ole Marse’. They all fear what will happen now that Preston is the Master. And indeed he has accused Harriet’s father, Ben, of helping runaways escape and that he must therefore know the identity of Moses. Ben is hauled off to jail where he will stand trial. Rit, Harriet’s mother, laments bitterly. In desperation she pleads for somebody to go to the town to find Moses and elicit his help.
Morning comes and “Moses” arrives to lead her family to freedom. All acclaim Harriet as they now recognize her as the famous Moses.
Harriet thus rescues her family, is reunited with Josiah and they all manage to reach the Canadian frontier. But as they cross the bridge to freedom, suddenly Preston and the slave patrol arrive in pursuit. Josiah steps in front of Harriet to shield her, and when Preston shoots, it is Josiah who is hit. Josiah with his last strength stumbles across the bridge and dies in Harriet's arms.
All her family and her people gather around to comfort Harriet and to mourn Josiah's death. They will not let her grieve alone. Nor will she fight alone. They have found freedom with her help and others must too. They will all fight together "so that no one is a slave and so that all can live together in peace, in harmony and in freedom."