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

When a young dockworker is found dead, the team discovers that there are mob connections; Jane tries to avoid sensitivity training; Maura starts spending time with Jane's brother.

When a warehouse fire kills a firefighter, Jane and Maura must investigate whether it was arson or an accident. But the investigation hits a snag when a hit-and-run leaves someone in the hospital. Guest star Billy Burke returns as FBI Agent Gabriel Dean, who hopes to rekindle his relationship with Jane. And guest star Jacqueline Bissett reprises her role as Maura’s mother, Constance Isles.

Jane and Maura’s friendship suffers following the shooting of Maura’s biological father, gangster Paddy Doyle. Jane also finds herself in the middle of an Internal Affairs investigation. And her relationship with Agent Dean is exposed.

Jane is unhappy when a former colleague returns as the new lieutenant of the drug unit; a tip from Cailin causes Maura to investigate Hope.

On the first day of Paddy’s murder trial, Maura gains insight into her father from Hope. Korsak opens up about Cavanaugh’s tragic past. And in an unexpected twist, Maura must decide which side she’s on when she and Jane unravel a shocking secret about Paddy and Hope that affects them all.

Jane and Maura are brought into the world of Keltic writings by the murder of a rare bookseller; Angela's relationship with Cavanaugh changes and Maura prepares Jane to meet her new man.