Faith Ringgold's second children's book follows Cassie, the main character of Tar Beach, and her baby brother Be Be on their journey through the Underground Railroad. Their journey started when they were flying one night and encountered a train in the sky. A woman wearing a conductor's uniform came out and called "All aboard!" Suddenly hundreds of people appear and get on the train, including Be Be. Cassie didn't want to go on the train and was frightened Be Be would never return after the train disappeared. The conductor's voice introduced herself as Harriet Tubman, called Aunt Harriet, and told her that the train would lead the slaves to freedom.
Aunt Harriet told Cassie about slavery and how slaves were treated. Then she informed Cassie that she would have to follow her brother on foot to Canada to see him again. She would have to use a passage that many slaves followed in the Underground Railroad. Cassie began at a slave plantation and had to follow a trail through the woods quietly to avoid being caught by bounty hunters. Along the way Cassie reached safe houses and other "conductors" who would direct her to her next destination. Along the way Aunt Harriet would sing a song of Moses to signal Cassie to keep going. There was a house with a quilt decorated with a star on it symbolizing it was a haven for escaped slaves. While Cassie hides in a cemetery she sees her brother's toys and baseball card nears a young boy's grave and starts crying, but Aunt Harriet whisper's her the next direction.
Cassie arrives at a yellow house in New York and receives a sign saying she is a free born girl from a railroad helper. She then hides out in a secret passage at a shoemaker's house until she gets transported in hidden compartment of a bookbinder's wagon. Once she is let out a funeral parlor, an undertaker hides her in a coffin and a gives her a ride on his hearse to Niagara Falls. When Cassie stops at Niagara Falls she regains her flying ability and is greeting by her brother, who is carrying a newborn baby he helped on his journey named Freedom, and Aunt Harriet along with many women dressed in white rejoicing in their safe reunion and escape. Cassie and Be Be hug each other and Be Be tells Cassie he now knows what their great-great-great grandparents went through. There was a huge celebration and feast for Aunt Harriet on the 100th anniversary of her leading her people to freedom.