As a young boy, Iosa Geraghty talks, plays, and lives with his family's farm animals, until at the age of nine, he goes to work driving the horses pulling the boats that carry freight up and down the 400-mile Erie Canal. After some childhood adventures with his friends, Iosa has a falling out with his own family and runs away from home to live on the canal full time.

In the city, Iosa meets one of New York's red-shirted firemen who introduces him to the city's vicious underside, as well as a young prostitute named Sibhon. The pair sell Iosa to Captain Caleb Black, skipper of a bogus canal chapel and the unscrupulous proprietor of a refuge for homeless drivers. Black, in turn, plans to sell Iosa and five other youths to prostitution houses in Buffalo and Chicago.

Not long after Iosa's pet mare, Alice, helps Iosa and his friends escape, the sinister slave broker Black and his mate are accused of murdering a young boy. They become the focus of a bizarre manhunt, but the renegade captain eludes authorities. Iosa's intuition and Alice's nose might be just what the law needs to bring Black to justice.