"Terra Incognita" is the 20th episode of season 4, and the 88th produced hour of Person of Interest. It originally aired on April 14, 2015.
Reese finds himself on a perilous personal journey when he takes on a homicide case Detective Carter was never able to close.
Origin of the Title
The Latin term terra incognita (lit: unknown land) is a cartographic term referring to unexplored or uncharted territory. As the globe was explored, early maps, labeled in Latin, marked those parts of Earth as yet unmapped as terra incognita. The expression and its English translation are also used as metaphors for a new experience or a journey into the unknown. Shakespeare famously called death "the undiscovered country from whose bourn no traveler returns" (Hamlet).
Person of Interest: Chase Patterson, a wealthy young man believed to have killed his parents. The murder is originally investigated by Carter, then lands on Reese's desk as a cold case.
This episode uses a more complex structure of flashbacks, memories and eventually hallucinations than we generally see in an episode, most from Reese's rather than the Machine's or Samaritan's point of view. Reese revisits his relationship with Carter through a series of memories and hallucinations, which return him to one evening early in Season 2 where he and Carter are on a stake-out monitoring a member of HR. The episode begins with Reese remembering their conversation during the stake-out. Later, after he is shot and left to freeze to death, the memories morph into hallucinations as his body temperature and heart rate decline, affecting his thinking. The first indication Reese has begun to hallucinate is when Carter refers to "your police shrink."
Also woven through the episode are a series of flashbacks detailing Carter's original investigation of the case with Terney, which parallel Reese's present-day investigation. Because these are not in MPOV, they appear to be Reese trying to imagine what Carter would originally have done based on his knowledge of her investigative style and what he finds in the case file.
The cold weather was both Reese's friend and his enemy. The extreme cold brings on a condition known a hypothermia, where the body begins to shut down as the result of prolonged exposure and subsequent loss of body heat. Among the first effects of hypothermia is the slowing of Reese's heart rate and the constriction of his blood vessels, which in turn slows blood flow and reducing bleeding. However, as the body's core temperature lowers, mental confusion sets in, resulting in his hallucinations. In addition, muscle coordination is reduced, respiratory rate slows, and blood pressure drops. In the final stages, amnesia sets in and the body's internal organs begin to fail. Despite this, an individual with hypothermia may survive even if they appear to have died; medical warming procedures increase survivability from hypothermia.
Reese was unable to drive away once he'd gotten into the car because, as Carter noted, "Cars don't like cold weather." As weather reaches freezing temperatures, the performance of a car's oil and battery changes, ultimately making it impossible to start the car. Both Reese's and Gil's cars had been sitting in the freezing weather long enough to make it impossible to start them without the help of a block warmer or other equipment designed warm the car's engine before any attempt to start it. On the other hand, Fusco's car would have been kept in a far warmer garage, making it possible for him to start his car and reach Reese.
The episode never makes it clear whether the POI lived or died. However, Gil stated the drugs administered to Chase gave him eight hours to live. Reese would have died of hypothermia much faster than Chase, but was found alive, making it reasonable that the writers meant us to understand that the POI survived.