911 gets a call from a male reporting a murder that's just about to happen. Using case-to-case DNA evidence, they determine the murder victim is the previously unidentified suspect know as the "Prospect Park rapist". Given the evidence, Mac and the team start questioning victims of the rapist to see if any of them have male relatives who might want vengeance. Trace evidence suggests one of the rape victims was present at the murder, however, she gets a lawyer quickly and refuses to answer any questions. When another sex offender is killed while their first suspect is in custody, our CSIs start expanding their search. The team then determines that their "male" caller/killer was using a voice modulator and it was actually a female caller. They are able to reverse the modulation and get a voice print of her two calls which they then match to 911 call recordings. It's found that the two killers (one a lawyer who used to defend rapists) met at a rape counseling center and decided to work together to seek vengeance on behalf of their fellow victims.