1867, Philadelphia. Amateur naturalist Walter Ash is on the brink of setting off to travel up his beloved Amazon when fate intervenes, obliging his only son to take his place. More at ease among his books than in the field, Paul Ash takes a reluctant leave of absence from Harvard's Museum of Comparative Zoology to accompany his grieving stepmother and her young companion to the fabled River Sea. Paul holds no memory of the place, though he was born there; he was still an infant when his father carried him out of the jungle and away from the mixed-blood family he might have known. As it transpires, however, neither the region nor its people have forgotten Paul. The Amazon lays claim to him in no uncertain terms, but it also works a peculiar magic on both his father's lovely widow and her friend—a quiet little Quaker named Rachel Weaver who proves strangely at home in the wild.