The killings on Isle Royale have left Anna Pigeon drained and haunted, her memories of her time with the wolf study group forever marred by the carnage on the island. Diagnosed with post-traumatic stress disorder, she is on administrative leave. Anna wonders if the leave might not be permanent, either by her own choice or that of the National Park Service.

The one bright spot in Anna's life is Paul, her husband of less than a year. Hoping the warmth and the adventure of a raft trip in Big Bend National Park will lift her spirits, Paul takes Anna to southwest Texas, where the sun is hot and the Rio Grande is running high. The sheer beauty of the Chihuahuan Desert and the power of the river work their magic -- until the raft is lost in the rapids and a young college student falls overboard, resulting in an even more grisly discovery. Caught in a strainer between two boulders and more dead than alive, is a pregnant woman, hair and arms tangled in the downed branches. Instead of a soul-soothing experience, Anna and Paul find themselves sucked into a web of intrigue that leads from the Mexican desert to the steps of the Governor's Mansion in Austin, Texas.