The killings on Isle Royale have left Anna 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, per her superintendent's urging. 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.