Harold Henthorn makes a harrowing 911 phone call, reporting that his wife Toni is gravely injured after falling off a cliff on a remote wilderness trail. By the time a ranger arrives on the scene, Toni is dead. After a routine investigation, suspicious details emerge, including that Harold's account of the accident is inconsistent and contradicts itself. Special Agent Beth Shott of the Investigative Services Branch (ISB), the detective force of the National Park Service (NPS), starts her work on the case. At the same time, authorities begin to receive anonymous letters and phone calls, leading them to reopen an investigation into the death of Harold's first wife, Lynn Rishell Henthorn, who died 17 years prior.