A Swallow's Heart is a story about faith, family, and friendships based in the 1860s, pre- and during the Civil War. The story is set in a fictional community called Pecan Grove in East Texas, and it follows two families, the Smiths and the Johnsons.

Their children, Mert and Kate Smith and Jonas and Jim Johnson, become close friends growing up. The two pairs of siblings enjoy adventures, including playing on the church grounds, riding horses on the Johnsons' ranch, and hunting in the woods that surround the Smiths' farm.

But when the Civil War starts, the kids go through a hard time when sixteen-year-old Jonas decides to go and fight for the Southern forces. Fifteen-year-old Mert has a very hard time with this, with Jonas being one of her best friends. She knows that he will be in great danger and that he might not return home.

The story focuses more on what happened at home during the war--the feelings and fears that the families and friends of the soldiers feel. But at the end, the story recalls a certain event that the foursome had shared together, and it shows that the ones that left left their hearts behind with their loved ones at home.