Thirteen-year-old Libby is a spoiled young lady who has spent the past four years living with her wealthy aunt. The year is 1857, and Aunt Vi tells Pa she can no longer manage Libby. Pa takes his daughter to live with him on the steamboat he captains. The boat is called the Christina after his deceased wife. He promises they will look after each other and be a never-give-up family.
Libby is both frightened and intrigued by her new life aboard the Christina. Her room is a third the size of the one she had at Aunt Vi’s. Against her wishes, Father buys her a huge Newfoundland dog named Samson for protection. She also has mixed feelings about the cabin boy, Caleb. Just a year older than she is, he lives aboard the boat with his grandmother, the cook. Libby sees shadows sneaking around the ship at night and suspects Caleb of smuggling people aboard.
When Pa makes Caleb chaperone Libby on a stop in St. Louis, Libby gets her first glimpse of the realities of slavery. They watch a slave auction in which a boy about their age, Jordan, is sold to a notoriously cruel slave trader named Riggs. Caleb whispers something to Jordan about an upcoming port of call for the Christina.
The next day, Libby believes she sees Jordan aboard the boat. When she learns she’s right about Caleb’s activities, she’s torn between supporting him and recognizing that her father could be imprisoned if slaves are found on his boat. When she tells Pa what she’s seen, she learns he and Caleb already share this secret and a code of honor based on their faith. Pa agrees to let Jordan stay on the boat for a while so he can get to Iowa and search among the Quakers for his family. Jordan believes it is his job to help get his mother and siblings to freedom.
Riggs, who has been scouring the river towns in search of his lost slave, sees Jordan at a port city. There, Caleb, Libby and Jordan borrow a wagon from Caleb’s old pastor. They hide Jordan in back and drive to the farm owned by some of Caleb’s Quaker contacts. Riggs almost catches them on the way, and he comes to search the Quaker home in the dead of night. Jordan hides beneath the floorboards. The Quakers tell Jordan where to find Emma, a friend of his mother’s, who has also escaped with her newborn son. Emma joins Libby, Caleb and Jordan.
They discover their escape route is blocked by Riggs and some of his men, so they know they must create a diversion. They also feel there’s a better chance Emma and Jordan can get past the slave traders without Emma’s baby. They form a plan in which Libby, posing as a girl who takes in laundry, sneaks the baby past the traders and back to the Christina. Caleb, Emma and Jordan return to the boat later.
Emma heads to Canada and freedom with her son. Jordan realizes he must stay on the Christina a little longer before he can rescue his family. People working with Riggs are already watching the farm where Jordan’s family lives, expecting him to show up. Libby gains Caleb’s and Jordan’s trust, and her father expresses his pride in her willingness to help slaves escape to freedom.