Jane looks for a way for Maura to meet her biological mother, Dr. Hope Martin. Angela gets involved in a political campaign. And a nun from Jane and Frankie Jr.'s past returns.

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.

Hope is ready to talk to Maura about a kidney for Cailin. Jane pays an unannounced visit on Casey. A substandard building related to a murder investigation endangers Jane's friends.

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.

The decades-old death of a mother and child open a closet door revealing skeletons that threaten to complicate the start of Paddy Doyle's RICO trial.

When a food truck chef dies suddenly, all signs point to a suspicious poisoning, but many questions remain unanswered. Meanwhile, Tommy announces some big plans that have Angela very excited. And Maura's birth mother, Hope, takes a bold step toward making amends with her daughter.

Jane works the case of an apparent accidental fall down the stairs of an affluent lawyer. Maura gets closer to Hope while recovering from surgery. Frankie and Nina's relationship comes to light.