Jane and Maura go undercover at a sex party to track down the perpetrator of a sex related murder in a mansion that's on the market. As Maura and Hope get better acquainted their relationship turns out to be especially important for Hope's daughter Cailin. Meanwhile, Sister Winifred tries to maintain some level of civility in the squad room.

In the season premiere, a community parade turns deadly and family tensions and politics arise. A surprise visit from Lt. Col. Casey Jones (CHRIS VANCE) reveals shocking news.

Jane and Maura investigate the death of a young woman who is found washed up on the beach. Maura confirms that she drowned and manages to trace the exact location by analyzing the water in her lungs: a tank in an old fish factory. Jane is somewhat upset to find that an old flame, police Lt. Rafael Martinez, is back in Boston. They had worked together in the narcotics division but their relationship ended after a confidential informant had been killed and Martinez left to join a special task force some 8 years ago. The dead girl is identified as Brenda Thompson, a journalism student who was pursuing a drug story on her college campus. When Jane finds Lt. Martinez's business card among her possessions, it seems that he was using her to get information. Meanwhile, Maura learns that she may be in danger owing to her father's activities.

"Sports rage" and angry hockey parents are the focus of the investigation of the murder at a youth hockey game.