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When a series of murders begins mirroring the infamous Boston Strangler killings, Danny and Lena race to stop a copycat killer 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 killer. 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; 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 killer. Meanwhile, Sarah reopens a troubling case from her past and Sean and Jonah get caught in the middle of a neighborhood feud.