Martin Brown breaks into Nina Previn's house and strangles her to death. She manages to call 911 before she is attacked and names her killer as Brown. Her body is discovered by Nash and the SIU the next morning. The discovery has a particular impact on Michelle because it was Michelle who convinced Nina to testify against Brown.
Brown is a murderer of three people who has escaped from prison. Nina's husband, surgeon Dr. Justin Previn vents his anguish out on Michelle. Justin reveals that Nina was the only eyewitness who could identify Brown during a home invasion at their home five years ago. Michelle feels badly, but Nash orders Michelle not to cancel her best friend Katherine's pre-wedding celebrations.
Meanwhile Joe has been trying to get Nash interested in his latest off-duty case involving Channel 8 reporter Grace Lopez. Grace is being blackmailed by someone who has stolen her private sex tape she made with her boyfriend. Nash is not interested but Nash gets Evan to partner with Joe on the off-duty case.
A former girlfriend of Brown, a boxer named Tysa, vows to kill Brown if he comes near her. Tysa leads the SIU to Astrid Dwyer, who has carried on an affair with Brown while Brown was in prison. Brown visits Astrid looking for money. A search of Brown's prison belongings reveals his obsession with getting revenge upon Michelle. As Nash and Joe rush towards Michelle's apartment, Brown is already there removing Michelle's back-up gun. Brown attacks Michelle and tries to strangle her as Nash and Joe intercede. For Michelle's safety, Nash orders Michelle to stay at his apartment.
The weird Dwyer family is brought into headquarters for questioning on the whereabouts of Astrid. But the stepfather, mother and daughter cannot reveal anything. Astrid calls Michelle during Katherine's bachelorette party complete with a male stripper held at Nash's apartment. Astrid sets up a meeting with Michelle so that Brown can have an opportunity to kill Michelle. Michelle agrees to meet without telling the rest of the SIU. A shoot-out occurs at the meeting. Michelle manages to wound Brown in the shoulder as he escapes in a police car.
In custody, Astrid refuses to reveal the whereabouts of Brown. Nash takes back Michelle's black pearl bracelet which Astrid is wearing, but he must check in the bracelet as evidence. (The bracelet was taken by Brown during his invasion of Michelle's apartment). Nash warns Michelle not to go off on her own again without informing the rest of the SIU.
Brown follows Michelle, now accompanied by Nash for her protection, to a bridal gown shop where Catherine is having her gown fitted. Brown kidnaps Catherine. The stolen police car is discovered near the gym where former girlfriend Tysa works out. Tysa and the gym manager are found tied up, but the manager's car is equipped with an auto theft tracking device. Brown is arrested in the stolen car and taken into custody.
Meanwhile Joe is exasperated with Evan. Evan has blown an undercover stake-out to meet with the blackmailer of Grace Lopez. Evan inadvertently gives Grace a cup of coffee causing the meeting to be canceled. On the second try, Joe hides in the trunk of Grace's car. Joe is in the trunk to greet the blackmailer as he comes to get the money. Things don't go as planned when the blackmailer takes the car. But Evan stops the car and they arrest the blackmailer who turns out to be Grace's pool man.
Nash interrogates Brown who taunts them with his kidnapping of Catherine. Brown refuses to reveal the whereabouts of Catherine, instead daring Nash to hit him and needling Michelle as being responsible for Catherine's death. Nash and the SIU brainstorm and center their search on a rail yard, where Catherine is finally found. An irate Nash calmly beats Brown in his holding cell. Dr. Justin Previn is satisfied as a bruised Brown is taken back to prison. Nash visits Michelle, returning her black pearl necklace found on Astrid's wrist.
For more, see commentary for Episode 40