Detective Casey Shraeger is a New York City vice cop who gets transfered into the homicide branch of the 2nd Unit, which is a "special unit" of detectives investigating crime and corruption in the city after one of the 2nd Unit detectives is found murdered. She is called in as his replacement to find out if the late detective was dirty. Casey begins to find out some interesting facts about her new co-workers, one of whom includes Det. Walsh, whose partner was the victim.
Casey Shraeger and her new partner, James Walsh, try to catch a family of criminals that has the entire 2nd Unit running all over New York until Walsh and Shraeger discover their motive. Meanwhile, Sgt. Brown has Shraeger investigate Walsh's past when it becomes known that her former partner turned out to be a dirty cop. Also, Henry Cole's former partner-in-crime returns, making it even more difficult for Cole to continue to hide his criminal past from the rest of the detectives.
Walsh's old cop friend is in danger of being charged with a murder he might not have committed. Meanwhile, Banks and Delahoy are trying to track down a man who has been stealing other people's identities.
Shraeger & Walsh are assigned protective duty of a businessman, but he escapes or is kidnapped while they step away to assist a woman being attacked. Delahoy & Banks deal with a nursing home resident with dementia who goes on a crime spree. Delahoy continues to have symptoms of his brain tumor.
A pair on con men trick Alvarez and steal important evidence for the drug trial of a dentist. Meanwhile, Bank's locks himself in his apartment due to paranoia and becomes convinced that his neighbor was murdered.
Delahoy and Banks investigate several break-ins that are related to an adult film series. Meanwhile Scraeger attempts to help serial crime accuser who may have been a victim of an attack ten years ago.