Deacon Fuller sends his son, Tommy, home early from church for misbehaving. Alone, Tommy goes to the barn to visit the horses, but when he hears a noise in the loft, he discovers a family of runaway slaves hiding in the hay. Big Jeff, his wife Vinnie, his son Little Jeff, and Baby Pearl, have escaped from a plantation in South Carolina and are traveling north to Canada where they can be free. They are following the drinking gourd, a constellation in the northern sky that points the way to freedom. Tommy learns that his father has hidden the family in the barn, and will be taking them across the river that night.

When Tommy's father comes home, he and Tommy hide the runaways in a hay wagon and drive them to the river. Along the way Tommy's father explains that he is a part of the Underground Railroad, a secret group of people who help runaway slaves escape to freedom. When they reach the river, Deacon Fuller leaves Tommy with the wagon and the runaway family, hidden in the hay, while he goes to get a boat.

While Tommy waits for his father to return, a search party looking for the runaway slaves finds Tommy and the wagon. They are about to search it when one of the men recognizes Tommy as the boy who got in trouble in church. Thinking quickly Tommy tells them that he is Deacon Fuller's son, and being afraid of beind punished by his father is running away. The men leave, deciding that they are on the wrong trail.

Deacon Fuller returns with the boat and rows Big Jeff and his family across the river, where a guide is waiting for them. Tommy is sent home, where he waits until he hears his father return. Deacon Fuller then tells Tommy that, although he believes one should always obey the law, he can not obey a law that says one man has the right to make slaves of other men. Tommy lies in bed, thinking about the runaways, and looking out his window at the drinking gourd.