As word of the discovery of gold in northern California spreads, an English physician, Nathaniel Trennant, accepts an offer to serve as doctor on a ship carrying immigrants to America. Alongside some two hundred emigres from northern Europe is a contingent of wealthy British people who call themselves not immigrants, but colonists. With this imported cultural divide, Nathaniel begins a long journey into what he terms the American experiment, one already under the stress of social injustices and economic inequities.

Trennant arrives in Baltimore and stumbles onto its slave market, where he witnesses the horrors of human bondage. One night in a boardinghouse he discovers under his bed a runaway slave. Disturbed and angered by the selling of human lives, he offers to help the young man escape, a criminal action that will put the slave and physician into flight from both the law and opportunistic slave hunters.

Travelling by foot, horse, stage, canal boat, and steamer, the two men gradually form a bond as they explore the backcountry and forge a deep friendship as they encounter the land and a host of memorable characters who reveal the nature of the American experiment.