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.

As Special Agent Shott investigates the suspicious death of Harold’s wife Toni, anonymous letters and phone calls lead authorities to reopen an investigation into the death 17 years earlier of Harold’s first wife, Lynn Rishell Henthorn, in what had appeared to be a freak accident on a remote mountain road. Shott and two of the original investigators on the case mount a new inquiry into the incident.

As the investigation into the suspicious death of Toni continues, Special Agent Shott learns something startling about Harold’s unusually close relationship with his sister-in-law. As the case grows in complexity, Shott is joined by FBI agent Jonny Grusing, who specializes in serial killers.

A search of the Henthorn home leads investigators to expose one of Harold’s biggest secrets. Shott and Grusing, her FBI partner, are confident that Harold murdered Toni, but they must assemble an airtight case with every scrap of evidence they can find. Criminal profiler Bryanna Fox provides additional insights into Harold’s background and behavior, leading investigators to worry that Harold and Toni’s young daughter, Haley, could also have been in danger.