An elderly couple and then a young girl become ill with typhoid fever on the wagon train. A young servant girl has attended to them all in isolation. One of the women on the train has an idea about the cause but no one wants to listen.

The Perez family is run out of town because people believe their daughter Juana is a witch. The family joins the wagon train, and after Juana's brother Felipe is mauled by a cat, the old world is pitted against the new world of medicine.