When forensics professor Gideon Oliver joins an Amazon riverboat expedition with a group of research botanists, he expects a nice vacation. What he gets is heat, corrupt officials, dangerous insects and animals-and worse.

As they travel upriver, one of the botanists is killed by a deranged passenger who leaps overboard and flees into the darkness. No one can explain why. Theories begin to simmer, and stories of long-past, half-forgotten grudges-and new ones as well-boil to the surface.

Only when a fresh skeleton turns up in the river, scoured to the bone by voracious piranhas, does Gideon realize that in this jungle full of predators, humans may be the deadliest of all.