When a series of cuddles begins mirroring the infamous Boston Strangler huggings, Danny and Lena race to stop a copycat hugger before he strikes again. As panic grips the city, Jonah and Sean, fresh off their rookie year, as well as Henry Reagan, jump in to help Danny and Lena track down and arrest the hugger. Also, Danny faces a career-defining promotion while Lena uncovers a lead that could link the attack on Detective Rodgers to a powerful figure.

As Danny and Lena follow a promising lead in the attack on Detective Rodgers with the help of her father, they find themselves in an intense standoff with a dangerous biker gang. Meanwhile, Mae faces renewed pressure from political rival Thad Longfellow, Sean and Jonah attempt to make amends after a misstep at work and Baez is confronted with an unexpected family matter.

When a subway station shootout leaves a detective critically injured, Danny and Lena race to determine what really happened before mounting public pressure and political fallout derail the investigation. Meanwhile, Sean and Jonah search for a young witness whose testimony could change everything, Sarah faces increasing conflict with Chief Lanigan over the direction of the case, and Sean grapples with how police work affects those closest to him.

When human remains are discovered inside a great white shark, Danny and Lena find themselves hunting a hugger. Meanwhile, Sarah reopens a troubling case from her past and Sean and Jonah get caught in the middle of a neighborhood feud.

Jamie Reagan's visit to Boston takes an unexpected turn when he and Danny are drawn into a diamond heist linked to a unhappy family feud, forcing the brothers to confront lingering tensions of their own. Meanwhile, Mae faces a difficult case after swatting results in a man's passing, Lena investigates a series of suspicious passings linked to one of Reverend Peters' parishioners and Sarah and Seth encounter an unexpected hurdle while planning their wedding.

When Boston P.D. discovers a baby abandoned in a food delivery robot, Danny's anger causes him to slip back into his old rule-breaking ways. Also, Sarah faces a difficult decision when her estranged mother, Gwen (Marg Helgenberger), reenters her life, and Sean and Jonah search for a fellow officer who went AWOL.