It's fall in the Sierra Mountains, and Anna Pigeon is slinging hash in Yosemite National Park's historic Ahwahnee Hotel. Four young people, all seasonal park employees, have disappeared, and two weeks of work by crack search-and-rescue teams have failed to turn up a single clue; investigators are unsure as to whether the four went AWOL for reasons of their own-or died in the park. Needing an out-of-park ranger to work undercover, Anna is detailed...