Jessie thinks Daniel is an escaped slave because he lives in isolation in the forest, nowhere near other people.
Jessie is a boy who plays the fife to help support his family until he is kidnapped by slave traders. He is told to make the slaves dance with his fife, but he hates the work and is just as much a slave as they are.
After a shipwreck, Jessie is able to go ashore. He meets Daniel, and old man who lives deep in the Mississippi woods.
I knew by then he must be an escaped slave who had founded for himself this tiny place of liberty deep in the forest. Often I felt we were as remote from other people as we would have been on a deserted island. (ch 8)
Daniel arranges for Jessie to take another slave boy named Ras to freedom. After he gets North where there is no slavery, he becomes an apothecary and sends for his family.
Before he was kidnapped, Jessie was curious about slavery. After his time on the ship, he went from hating the slaves to identifying with them. When he finally got away, he wanted to go somewhere where there was no slavery.