Chase and August receive news that they've been selected for promotion to Sergeant, but the celebration is put on hold when the mayor is brutally gunned down. When their arrest of the shooter brings in tough-as-nails Internal Affairs officer Samantha Cullen, the detectives realize they've stumbled onto something bigger than just murder. As they begin to uncover a plot the department has been trying to keep under wraps, their investigation leads them to Joshua Riggs, an ex-cop who has a past history with Chase.