At the beginning of the season Sylvia becomes pregnant with Andy's child. A baby boy, Theo, is born towards the end of the season. This is contrasted with the fate that awaits Sipowicz's older son, Andy Jr., who announces that he is to join the police force. Andy is finally bonding with his estranged son when he is gunned down, which leads the elder Sipowicz to fall off the wagon. Andy Jr's murderers are killed themselves by Simone in an act of self defense.
Bobby and Diane, whose relationship had been put on hold while she attended AA, resume their relationship only for Diane to begin drinking again when her abusive father beats her mother. Her father is eventually killed and her mother becomes the prime suspect.
James Martinez and new detective Adrienne Lesniak begin an affair but only after Lesniak tells Medavoy that she is gay, then that she is not; Martinez later breaks up with her due to her controlling and unpleasant behavior, and Lesniak eventually leaves the squad. Medavoy himself leaves his wife, recognizing that she is holding him back but it is too late to save his relationship with Donna who leaves to take up a job with Apple.
January is forced to return home after six years traveling abroad. A near-fatal accident has left her temporarily wheelchair bound and depleted of her long-term memory. She is accompanied by her boyfriend 'Callum' whom she met whilst traveling. He was also the driver in the fateful accident. Not only has she forgotten her family, but her childhood as well and is surprised to discover that her home is a stately manor in the middle of the country-side. As January tries to settle in, she finds herself becoming even more estranged from her family, who just want their daughter back. The trouble is, she cannot remember who that daughter was, or why she ran away in the first place. Desperate to discover why she left all those years ago and with Callum's help, she sets out to find the truth. They soon discover the family are not as loving as they seem to be. Was there a dark reason why she left them in the first place?