“Key” follows Selin, an intelligence officer, and Erol, a chief inspector from the homicide bureau, as they investigate the unsettling mystery behind the unexplained deaths of ten inmates in a high-security prison.

Ten inmates are found dead in their beds during a routine morning count at a high security prison. With no signs of strangulation, injury, or violence and no explanation from guards or prisoners, the facility is placed under quarantine. Selin Korkmaz from the intelligence unit and Chief Inspector Erol Haktan from the homicide bureau are assigned to uncover what happened. As they interrogate everyone inside and examine every detail, each locked door they open brings them closer to a truth more disturbing than anyone anticipated.