A woman is found raped and murdered in Central Park by a dog walker. The Medical Examiner concludes the victim was stabbed to death and sexually assaulted. The detectives have a hard time identifying the victim or finding any suspects. The woman's dog is found a while later, the dog had one of her shoes with it and due to an orthopedic heel in it they are able to fin out the victim's identity, she was a public health doctor named Greta Esterman.
Olivia and Elliot talk to her husband, he tells the detectives her work involved tracking sexual partners of patients recently diagnosed with HIV to have them checked as well. Mr. Esterman says his wife occasionally received hate mail and threatening calls, but never paid any attention to it. The detectives go through Greta's log of all the threatening calls and find one threat that matches the crime, it is from Roger Pomerantz. When Elliot and Olivia go to talk to Roger, he tries to flee, so he is arrested on violation of his parole. He is interviewed but has an alibi, so he is released; at that time, Mr. Esterman is arriving at the station to deliver some of Greta's work documents that might help in the investigation, they both cross paths and Mr. Esterman believing Roger is his wife's killer, looses control and punch him.
Dr. Warner analyzes Greta's papers and comes to the conclusion she was trying to identify the common source of the HIV from some her patients, she believed they all had the same source. The detectives think that maybe Greta was close to finding the person that was spreading the HIV, so that person killed her. They follow the list of people she tracked and interview people trying to establish who gave it to whom, trying to find the source, they believe it is Mario Molinari, since he can be connected directly or indirectly to all the infected people. Elliot and Olivia find Mario at a hospital and confront him about Dr. Esterman's death, it turns out he was in the hospital at the time, but Elliot arrests Mario for knowingly giving HIV to people, since he knew about his condition and still had unprotected sex without telling his partners about his HIV.
Meanwhile another body is found in the park, leading the detectives to think there is a serial killer on the loose, but the ME thinks maybe it is a copycat, since the injuries sustained by the second victim are not as many and not as and as the first victim. The forensics analyst disagrees; he found exactly the same evidence in both crime scenes, the same footprints and the same knife type. He also says that both types of soils are unique to that place, so of they catch a suspect, they can analyze his shoes, and match it to them giving no doubt about who killed those women.
Olivia and Elliot try to find a connection between the victims, so they talk to Greta's friend. She tells about Greta's marriage to Arthur, how her family was against it because she was rich and he wasn't, and how Greta was the one who supported the couple. Elliot asks if any of them could've had any affair, and she reveals Greta asked her to check a man's credit report, the man's mane was Vartan Dadian. The detectives investigate this Vartan guy and find out he had an account and received a steady amount of money from Greta's account every month, they also find charges to an escort service in Vartan's name. Munch and Fin go to the Escort agency and get the name of the lady that serves Vartan regularly, Miss Kitty.
Elliot and Olivia talk to Vartan Dadian about it, but he denies having an account in the bank in question, and claims he never met Greta or Miss Kitty. The detectives realize someone was using Mr. Dadian's identity to open the account and hire escorts, probably to hide from somebody. Munch and Fin interview Miss Kitty at the precinct and she recognizes Mr. Esterman's picture as being Vartan Dadian.
The detective get a search warrant for Mr. Esterman's home, trying to fin some solid evidence that can prove he murdered his wife when she found out about his affair and decided to divorce him, because if she divorced him he would be left with nothing. Munch finds a dirty pair of shoes in Arthur's bedroom that matches the shoe-prints found in the crime scene, the lab matches the dirt from the crime scenes and the dirt on the shoes. Mr. Esterman denies any involvement in the murders and claims he was in the park after the murders, to lay flowers there. He argues that he took an alias to meet with escort girls to protect his wife; when asked for a DNA sample, he replies his DNA is expected to be found in his wife, but Elliot reveals they are going to match his DNA against a hair found in the other victim's body.
The judge denies the warrant for Mr. Esterman's DNA, but Elliot finds out he already gave a DNA sample; in a fund-raising charity event, he donated blood, so they ask the judge to use that blood to compare to the sample in the victims body. They argue that he waved any right to privacy when he donated the blood, since it would be tested anyway in a hospital. The judge hears both sides and agrees to let the State test the blood, but only the same tests that would be done in the hospital to which he donated the blood. The test comes back and they find a high amount of Viagra and ecstasy, so they talk to Arthur's drug dealer, he gives up the address to where he delivers the drugs.
Munch and Fin go there with a search warrant and catch him in bed with Miss Kitty; there they find a one-way ticket to Brazil and Kitty gets furious when she realizes he was planning to go alone. Kitty gives the detectives a sample of Mr. Esterman's DNA and they match it to the hair from the second crime scene. Arthur Esterman admits killing his wife and the other woman to divert the detectives, he claims she drove him to it, when she lost her sexual appetite; he tried to make her try the ecstasy, but she refused so he started seeing prostitutes, she found out and was going to divorce him leaving him with no money for his ecstasy or the prostitutes, so he had to kill her.