A major theme of the story is the way the institution of slavery functions as a lens to explore the best and worst in human nature. Additionally, although the novel focuses primarily on American slavery, McBride also considers the ways slavery and freedom are mental and emotional states as well as legal and physical states.

Denwood is a slave catcher by trade, yet he recognizes that he himself is a slave to the inner demons brought on by denying freedom to the runaways he catches. He knows that what he does is evil and that his money is blood money, and this knowledge corrodes his psyche. Denwood realizes that like the slaves he pursues, he is also a runaway, in that he has been running away from the life he should have led—that of a poor but decent oysterman. At the end of the story, Denwood frees himself from the shackles of his mind by an act of selflessness.

Owning people acts as a mental trap for slave owners. Fear of uprisings and runaways forces slaveowners to obsess about their slaves and make them a central point around which their lives revolve. As Denwood says, slave owners cannot sleep until their slaves sleep, cannot stop working until their slaves stop, get up in the morning when their slaves do.