In October 1995, Andrea Lankford, a 31-year-old supervisory park ranger at Grand Canyon’s North Rim, searches 20-year-old Gabriel Parker’s abandoned vehicle for clues. She inventories the messy car, noting the belongings that suggest Parker was conscientious about litter. Parker had left Washington State in September, telling his roommate he needed to be independent.

As operations chief, Lankford coordinates a seven-day search, but nothing is found. She meets Parker’s father, Doug, and becomes emotionally invested, growing upset when the search is ended. Doug’s thanks only deepen her sense of failure.

Months later, maintenance workers discover Parker’s body at the base of the Redwall, a 300-foot escarpment. Colleagues concluded that he died by suicide; Lankford believed he had fallen while smoking cannabis. In 1999, overwhelmed by stress, she leaves the National Park Service, hikes the Appalachian Trail, and becomes a registered nurse.

In early 2017, television producers researching missing hikers introduce Lankford to the case of Chris Sylvia, a 28-year-old who vanished from the Pacific Crest Trail in February 2015. When the show is cancelled, Lankford promises Sylvia’s mother she will investigate. She soon learns that two other young men, Kris Fowler and David O’Sullivan, have also disappeared from the trail in consecutive years. Lankford begins an investigation that will unite amateur searchers in a difficult search.