Donna's younger sister, Dana, arrives and moves in with her and Medavoy, and tries to make the moves onto him. Simone doubts the confession of his friend's 11-year-old son, a murder suspect in a gang related shooting at a school. Meanwhile, an HIV-infected Lothario comes to the station and asks Lesniak and Martinez for help when he appears to be targeted by a woman he's apparently infected.

Sipowicz investigates the murder of a pimp in which evidence points to a cop that Sipowicz once helped out with a problem of his. Later, Simone mentions the case to his girlfriend reporter Benita, and soon regrets it when the story hits the papers which leads to the suspect committing suicide. Benita denies leaking the story, but Simone refuses to believe her and ends their romance. Meanwhile, Donna's former boyfriend, Harold, a ex-New York Ranger hockey player, returns to the city and meets with Donna, while Meadvoy overreacts with jealousy. Donna confides her situation with her sister, Dana, but she manipulates Donna by sowing seeds of anxiety into her about Medavoy and his neurosis. Elsewhere, Martinez begins to develop feelings for Lesniak.

Simone and Sipowicz collide in their squad car with a Romanian immigrant who has a woman locked in the trunk of his car and later, their investigation shows him to be a possible terrorist and holding a wealthy family prisoner with pipe bombs. Meanwhile, the serial killer, George Putnam is shot by an unknown sniper. Simone begins to investigate the father of the killer's last victim, Mr. Rudman, and another member of a victim's support group... who all appear to be part of a secret vigilante group. Medavoy talks to Dana about his relationship with Donna if he's losing interest in her, but the crafty Dana takes advantage of the situation to try to break them up by playing on his mental neurosis as the cause of Donna's lack of trust in Medavoy. Also, Martinez keeps asking Lesniak to go out on a date with him.