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.