Tensions are high as the squad investigates a possible copycat of the legendary Boston Strangler…or could it be the Boston Strangler himself? Meanwhile, Jane butts heads with her new boss, Lt. Grant, who just happens to be from the same neighborhood where Jane grew up. And Maura gets into trouble with her habit of diagnosing everyone – including her date.

The Boston Marathon turns into a crime scene when a shooter begins targeting runners. Racing to catch the killer before another shooting, Jane and Maura must conduct their investigation in secret so as not to cause a mass panic.

An investigation leads Jane into Boston's checkered past when an identity thief is murdered; Maura goes on a personal quest.

After an undercover cop working a narcotics-smuggling sting is shot dead, the killers next turn their sites on Boston homicide headquarters, leaving Jane, Maura and Frankie Jr. trapped in a fight for survival on their home turf. It's a situation that will force Maura to challenge herself like never before. On the home front, Jane's parents are excited that their oldest child, Tommy, is being released after serving time in prison.

Tensions continue to mount between Jane and Maura, causing those close to them to be caught in the middle. But they must put aside their differences when they investigate the case of a college student who died in a tunnel. Meanwhile, Frankie Sr. returns with news that upsets Angela.

A musician is stabbed in a bar; the squad realizes that the stabbing victim had something to hide; Jane's father returns with shocking news.