The fourth season focuses on the search for a missing Navajo girl, which takes Leaphorn, Chee and Manuelito from the safety of Navajo Nation to the gritty terrain of 1970s Los Angeles in a race against the clock to save her from an obsessive killer with ties to organized crime.

Six months after the events of Season 2, Lt. Joe Leaphorn and Jim Chee investigate the disappearance of two boys, with only an abandoned bicycle and blood-stained patch of ground left in their wake. Meanwhile, Bernadette Manuelito attempts to settle into her new life 500 miles from home with the Border Patrol, but stumbles across a conspiracy involving human and drug smuggling with far-reaching implications.

On a bleak future Earth, a soldier endures a radical genetic transformation to save humanity. But his wife fears he's becoming more creature than man.