While processing a crime scene, Danny is suddenly attacked by a suspect hiding in the victim's closet. The ensuing chase leads them to a crowded New York subway where the suspect, an undercover cop and Danny all fire shots. When the smoke clears, the team is left with a dead cop, a missing suspect and Danny under investigation for the possible murder of the officer. Mac must prove who killed the cop in order to save Danny's job. Meanwhile, Stella investigates the death of a nanny killed in a Central Park restroom.