It's the summer of 1916, and 15-year-old Howard Leland Hutchings, his father, Dr. William Hutchings, and his 5-year-old twin brothers, Raymond and Earl, make the trip from St. Louis across the Mississippi River to Dr. Hutchings' hometown of Grand Tower, Illinois, in a Ford Model T touring car. They're going to visit Dr. Hutchings' parents and aunt and uncle, who all live together in a house on a hill called the Devil's Backbone. Which, yeah, creepy.
When they get there, Howard's father spends most of his time with his Aunt Delphine, who is bedridden and dying. The little boys hang out with their great-uncle Noah, a dude with one arm who keeps them entertained. It's the perfect family vacation for 5-year-old boys.
Old Dr. Hutchings, who is really old (not just regular old), spends most of his time napping in the sun. So, this leaves Howard to spend his afternoons talking with Tilly, Noah's twin sister, and Old Dr. Hutchings's wife. Howard's visit is the frame story for Tilly's story, and Tilly takes over as narrator for most of the book in order to tell the story of the first year of the Civil War and how all the old people came to live in that house together.
In 1861, the Pruitt family (except Paw, who abandoned them to work on the river) lives in the house on Devil's Backbone. The family consists of Mama, 15-year-old twins Noah and Tilly, and 12-year-old Cass. Into their lives come Delphine Duval and Calinda, two teenagers from New Orleans who mysteriously step off a steamboat and decide to board with the Pruitts.
When Noah goes to war, Mama sends Tilly after him to bring him home, and Delphine goes with her. They find Noah in an army camp in Cairo, Illinois, too sick to fight. As they nurse him, Delphine reveals that Calinda is her sister and that they are gens de couleur, or free people of color, from New Orleans. Women in her society have families and homes with white men but do not marry them, while the white men also have families with their white wives. Afraid that the coming war would destroy their fragile society, their mother sent them north to make new lives. Whoa.
Once he gets better, Noah fights in a single battle and gets shot in the arm. Dr. Hutchings amputates his arm, and the three teenagers return home, only to find that Mama drowned herself in the river when she saw a coffin unloaded on the dock. She thought it was Noah, but it actually turns out to be long-lost Paw, who fought in the same battle on the Confederate side. Dr. Hutchings serves throughout the war as an army doctor and then comes home to marry Tilly.
Howard takes over the narration at the end. Tilly tells him that Delphine and Noah never married, though they lived together all their lives. She tells him she fell in love with Dr. Hutchings through the letters he sent her over the years of war. Aw.
At last, the younger Hutchings head back to St. Louis. On the road, his father tells Howard that Delphine and Noah are actually his parents, but the four old people decided when he was born that he would pass as Dr. Hutchings and Tilly's son. Delphine was afraid people would find out she had African blood and be prejudiced against him. He says that really he had four parents and reveals to Howard that he plans to volunteer as an army doctor if the United States enters the Great War.