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.
Jane and Maura find a man who died in a bathtub full of ice; while trying to stay cool during a heatwave, Angela discovers a spicy tea that brings unexpected romantic consequences.
A notable county prosecutor is arrested for murder; when Angela hears that Maura is planning to meet Jack's daughter, she sets out to educate Maura about the interests of children today.
After Jane jumps from a bridge to save a prosecutor suspected of his murdering his mistress, Maura is terrified that she has lost her best friend. The squad must now renew the search to find the real killer. Meanwhile, Angela enlists Frankie's help as she applies to new jobs.
When a severed human foot washes up on shore, the squad is called in to identify the victim and track down the murderer as more body parts are found. After an unsuccessful job hunt, Angela finds she enjoys helping out Korsak at the Dirty Robber.
The squad is forced to join forces with an over-eager private investigator when a successful clothing designer is found poisoned and his wife reported missing. Jane notices Angela is spending too much money and asks Maura to teach her mom how to handle her finances.
After a man is found bludgeoned to death on the side of the road, Jane and the squad team up with another detective who believes he is hunting the same roadside killer. Meanwhile, Maura discovers she has had her identity stolen and recruits Angela to help track down the culprit.
The squad works overtime hunting down a serial killer sniper. Maura befriends a witness to the crime, who happens to suffer from a mental condition that has him believing he is a wolf.