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.

Baby TJ's christening turns grim when a body is discovered in the church. Jane almost abuses her power in order to protect her brother Tommy (COLIN EGGLESFIELD).

Dr. Isles becomes the suspect in the murder of a mysterious guest of a charitable fund-raiser.